Hi All

I have written a trigger for Apex sharing rule based on some criteria.But its showing error when other user creating account ..

AccountShare: execution of AfterInsert caused by: System.DmlException: Insert failed. First exception on row 0; first error: INSUFFICIENT_ACCESS_ON_CROSS_REFERENCE_ENTITY, insufficient access rights on cross-reference id: [] Trigger.AccountShare: line 34, column 1

trigger AccountShare on Account (after insert, after update) 
{
 if(checkRecursion.runOnce()){
    for(Account acc : Trigger.new){
        if(acc.Account_Manager__c!=null){   
            List<AccountShare> shares = new List<AccountShare>();
            Map<String,String> mParentID = New Map<String,String>();
            Map<String,String> UId = New Map<String, String>();
            system.debug('+++'+acc.Account_Manager__c);
            for(User usrmgr : [select id from user where id=:acc.Account_Manager__c]){
               AccountShare share1 = new AccountShare();
               share1.AccountId=acc.Id;
               share1.UserOrGroupId= usrmgr.id;
               share1.AccountAccessLevel='Edit';
               share1.OpportunityAccessLevel='Edit';
               share1.CaseAccessLevel='Edit';
               Insert share1;
               system.debug('+++'+share1.id);
            }
          
          for(User u :[Select UserRoleId from User  where id= :acc.Account_Manager__c])
          {
            for(UserRole ur : [Select ParentRoleId from UserRole where id=:u.UserRoleId]){
              mParentID.put(String.ValueOf(ur.get('ParentRoleId')),String.ValueOf(ur.get('ParentRoleId')));
              system.debug('+++1111');
            }
          }
           for(User u1 :[Select id from User u where u.UserRole.id in :mParentID.KeySet()]){
          
           //system.debug(mParentID);          
              for (Account rec : [SELECT Id FROM Account WHERE Id = :acc.id]) {  
               system.debug('+++2222');
               
                      AccountShare share2 = new AccountShare();
                     
                      share2.AccountId=rec.Id;
                      share2.UserOrGroupId=u1.id;
                      share2.AccountAccessLevel='Edit';
                      share2.OpportunityAccessLevel='Edit';
                      share2.CaseAccessLevel='Edit';
                      
                      Insert share2;  
                      
                      system.debug('+++'+share2.id);
                }                    
                    
            }
        }
    }
 }
}