I am getting an error LeadOwnerAssignment: execution of AfterInsert caused by: System.FinalException: Record is read-only

Question

trigger LeadOwnerAssignment on Lead (before update,after insert) { 
    if(Trigger.isAfter)
    {       
        List<User> userobj=[Select id from User where Profile.Name='Identity User'];            
        Integer Lead_Number;           
        for(Lead newLeads:trigger.New)     
        {                  
            Lead_Number=integer.valueof(newLeads.Lead_Number__c);
            System.debug('LeadNo'+Lead_Number);
            if(math.mod(Lead_Number,2)==0)             
            {
                newLeads.OwnerId=userobj[0].Id;
                System.debug('New lead owner Id'+newLeads.OwnerId);
            }        
            else
            {
                newLeads.OwnerId=userobj[1].Id;
                System.debug('New Lead Owner Id'+newLeads.OwnerId);
            }              
            newLeadFormedObject.add(newLeads);
        }            
        update newLeadFormedObject;                                   
    }    
}

Answer 1

You cannot update context variables in after triggers, you will have to use before trigger - https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_triggers_context_variables_considerations.htm

If you must need to use after trigger then you will have to fetch data using SOQL and then update its field values.

Answer 2

trigger LeadOwnerAssignment on Lead (before update,after insert) { if(Trigger.isAfter) {

             List<User> userobj=[Select id from User where Profile.Name='Identity User'];

                Integer Lead_Number;
                Map<id,Lead> leadMap=new Map<id,Lead>();
                List<Lead> newLeadListToUpdate=new List<Lead>();

                for(Lead leadObj :trigger.new)
                {
                    leadMap.put(leadObj.id,leadObj);
                }

                List<Lead> leadList=[select id,ownerid,name,Lead_Number__c from Lead where id in : leadMap.keySet()];

                for(Lead leadObj : leadList)
                {
                    Lead_Number=integer.valueof(leadObj.Lead_Number__c);
                    if(math.mod(Lead_Number,2)==0)             
                     {
                       leadObj.OwnerId=userobj[0].Id;
                       System.debug('New lead owner Id'+leadObj.Ownerid);
                     }

                     else
                     {
                      leadObj.OwnerId=userobj[1].Id;
                      System.debug('New Lead Owner Id'+leadObj.OwnerId);
                     }
                     newLeadListToUpdate.add(leadObj);
                } 
                update newLeadListToUpdate;               

            }

}