PDF generation failed. Check the page markup is valid. Issue with datatable and columns tags

Question

When rendering a visualforce page as PDF, I'm running into the error above often and have been unable to figure out what the error is.

Through process of elimination, I have been able to narrow down the problem sections to my use of the datatable and columns tags, but can't figure out what the cause of the problem is. I read in the developer notes that you have to use datatable with caution and if a column is blank that could be the error, but I'm not sure if that is my problem. If anyone can take a look at my code and suggest any fixes, that would be great!

Below are the datatables that are causing the error message to fire:

                        <apex:dataTable value="{!Opportunity.opportunityLineItems}" var="OP" style="width:98.7%; border:1.5px solid #d9d9d9; border-bottom: 0px !important;">
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'NQP'}" headerValue="TR Code" headerClass="column_ttl tac" styleClass="tbl_bdr tac" style="width:10%;"><apex:outputText value="{!OP.TR_Service_Group__c}"/></apex:column>
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'NQP'}" headerValue="{!lblQty}" headerClass="column_ttl tac" styleClass="tbl_bdr tac" style="width:5%;"><apex:outputText value="{0, number,###,###,##0}"><apex:param value="{!OP.Quantity}" /></apex:outputText></apex:column>
                        <apex:column headerValue="Description" value="{!OP.PriceBookEntry.Name}" headerClass="column_ttl" styleClass="tbl_bdr" rendered="{!OP.TR_Service_Group__c = 'NQP' && Opportunity.Language__c = 'ENGLISH'}"></apex:column>
                        <apex:column headerValue="Description" value="{!OP.Product2.Product_name_french__c}" headerClass="column_ttl" styleClass="tbl_bdr" rendered="{!OP.TR_Service_Group__c = 'NQP' && Opportunity.Language__c = 'FRENCH'}"></apex:column>
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'NQP'}" headerValue="Regular Price" value="{!OP.ListPrice}" headerClass="column_ttl tac" styleClass="tbl_bdr tar" style="width:15%;"></apex:column>
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'NQP'}" headerValue="Sold Price" value="{!OP.UnitPrice}" headerClass="column_ttl tac" styleClass="tbl_bdr tar" style="width:15%;"></apex:column>
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'NQP'}" headerValue="{!lblSubTot}" value="{!OP.TotalPrice}" headerClass="column_ttl tac" styleClass="tbl_bdr tar" style="width:15%;"></apex:column>
                    </apex:dataTable>          

                    <apex:dataTable value="{!Opportunity.opportunityLineItems}" var="OP" style="width:98.7%; border:1.5px solid #d9d9d9; border-top: 0px !important;">
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'CNT'}" headerValue="TR Code" headerClass="column_ttl tac" styleClass="tbl_bdr tac" style="width:10%;"><apex:outputText value="{!OP.TR_Service_Group__c}"/></apex:column>
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'CNT'}" headerValue="{!lblQty}" headerClass="column_ttl tac" styleClass="tbl_bdr tac" style="width:5%;"><apex:outputText value="{0, number,###,###,##0}"><apex:param value="{!OP.Quantity}" /></apex:outputText></apex:column>
                        <apex:column headerValue="Description" value="{!OP.PriceBookEntry.Name}" headerClass="column_ttl" styleClass="tbl_bdr" rendered="{!OP.TR_Service_Group__c = 'CNT' && Opportunity.Language__c = 'ENGLISH'}"></apex:column>
                        <apex:column headerValue="Description" value="{!OP.Product2.Product_name_french__c}" headerClass="column_ttl" styleClass="tbl_bdr" rendered="{!OP.TR_Service_Group__c = 'CNT' && Opportunity.Language__c = 'FRENCH'}"></apex:column>
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'CNT'}" headerValue="Regular Price" value="{!OP.ListPrice}" headerClass="column_ttl tac" styleClass="tbl_bdr tar" style="width:15%;"></apex:column>
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'CNT'}" headerValue="Sold Price" value="{!OP.UnitPrice}" headerClass="column_ttl tac" styleClass="tbl_bdr tar" style="width:15%;"></apex:column>
                        <apex:column rendered="{!OP.TR_Service_Group__c = 'CNT'}" headerValue="{!lblSubTot}" value="{!OP.TotalPrice}" headerClass="column_ttl tac" styleClass="tbl_bdr tar" style="width:15%;"></apex:column>
                    </apex:dataTable> 

                    <apex:outputPanel rendered="{!CONTAINS(strServiceType, 'PREPAID') && CONTAINS(Opportunity.Financing_Term__c, '36')}">    
                        <apex:dataTable value="{!Opportunity}" var="OPP" headerClass="hidden" style="width:98.7%; border: 1.5px solid #d9d9d9; border-top: 0px !important; border-bottom: 0px !important;">
                            <apex:column styleClass="tbl_bdr tac" style="width:10%;"><apex:outputText >HPX+HPS</apex:outputText></apex:column>
                            <apex:column styleClass="tbl_bdr tac" style="width:5%;"><apex:outputText value="{0, number,###,###,##0}"><apex:param value="{!OPP.TR_Prepaid_Unifon_Services_Qty__c}" /></apex:outputText></apex:column>
                            <apex:column styleClass="tbl_bdr tal" ><apex:outputText >Prepaid Services</apex:outputText></apex:column>
                            <apex:column styleClass="tbl_bdr tar" style="width:15%;" value="{!OPP.TR_Prepaid_Unif_n_Service_36__c}"/>  
                            <apex:column styleClass="tbl_bdr tar" style="width:15%;" value="{!OPP.TR_Prepaid_Unif_n_Service_36__c}"/>  
                            <apex:column styleClass="tbl_bdr tar" style="width:15%;" value="{!OPP.TR_Prepaid_Unif_n_Service_36__c}"/>  
                        </apex:dataTable>
                    </apex:outputPanel>

                    <apex:outputPanel rendered="{!CONTAINS(strServiceType, 'PREPAID') && CONTAINS(Opportunity.Financing_Term__c, '60')}">    
                        <apex:dataTable value="{!Opportunity}" var="OPP" headerClass="hidden" style="width:98.7%; border: 1.5px solid #d9d9d9; border-top: 0px !important; border-bottom: 0px !important;">
                            <apex:column styleClass="tbl_bdr tac" style="width:10%;"><apex:outputText >HPX+HPS</apex:outputText></apex:column>
                            <apex:column styleClass="tbl_bdr tac" style="width:5%;"><apex:outputText value="{0, number,###,###,##0}"><apex:param value="{!OPP.TR_Prepaid_Unifon_Services_Qty__c}" /></apex:outputText></apex:column>
                            <apex:column styleClass="tbl_bdr tal" ><apex:outputText >Prepaid Services</apex:outputText></apex:column>
                            <apex:column styleClass="tbl_bdr tar" style="width:15%;" value="{!OPP.TR_Prepaid_Unif_n_Service_60__c}"/>  
                            <apex:column styleClass="tbl_bdr tar" style="width:15%;" value="{!OPP.TR_Prepaid_Unif_n_Service_60__c}"/>  
                            <apex:column styleClass="tbl_bdr tar" style="width:15%;" value="{!OPP.TR_Prepaid_Unif_n_Service_60__c}"/>  
                        </apex:dataTable>
                    </apex:outputPanel>    

Answer 1

Please take a close look into the limitations of PDF generation in Visualforce pages documented here

If an <apex:dataTable> or <apex:pageBlockTable> has no <apex:column> components that are rendered, rendering the page as PDF fails.

To work around this issue, set the table component’s rendered attribute to false if none of its child apex:column components are rendered.

Hence make sure to not render the datatable if no apex:column components are rendered. For example, you can do below

 <apex:dataTable value="{!Opportunity.opportunityLineItems}" 
        var="OP" style="width:98.7%; border:1.5px solid #d9d9d9; border-bottom: 0px !important;"
   rendered={!Opportunity.HasNQPService__c} >

where HasNCPService__c is a formula field on Opportunity that is true if there is at least one opportunity line item that has TR_Service_Group__c = 'NQP'