This is my first question. I have some doubts about one trouble. I saw that it could be there  a duplicate question, but i'll try anyways. 

My vf page code is below:

    <apex:form >
       <apex:pageblock >
        <apex:pageBlockButtons >
        <apex:commandButton value="Cancelar" action="{!cancelar}" />
        </apex:pageBlockButtons>
       <apex:pageBlockSection>
        <apex:repeat value="{!mapaCantProductos}" rendered="{!IF(tempListaInnerProd.size != 0,true,false)}" var="ipId">
         <apex:repeat value="{!mapaCantProductos[ipId]}" var="oli">
            <apex:inputField value="{!oli.UnitPrice}" label=""/>
            <apex:inputField value="{!oli.serviceDate}" label="" required="true" />
          </apex:repeat>
         </apex:repeat>
        </apex:pageBlockSection>
       </apex:pageblock>
      </apex:form>

And the controller code is:

     public PageReference cancelar(){
     	PageReference pf = new PageReference('/'+this.opp.Id);
     	pf.setRedirect(true);
     	return pf;
     }

Then when i try to click on it, an inputField required error is displayed: "Must write a value" or something like that. I could read some about apex:ActionRegion tags to split up the form into various groups of fields with their associated actions. 

My inputFields within apex:form and apex:pageBlock are below:

     <apex:inputField value="{!oli.serviceDate}" label="" required="true" />


Thanks in advance, any help to handle this situacion will be helpful.