I had the same error, in the test class for the trigger SyncProductstoUser. I fixed it without @futurecall as per R Draper
System.DmlException: Update failed. First exception on row 0 with id 00318000008sC90AAE; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, SyncProductstoUser: execution of AfterUpdate

caused by: System.DmlException: Update failed. First exception on row 0 with id 00518000000MvpVAAS; first error: MIXED_DML_OPERATION, DML operation on setup object is not permitted after you have updated a non-setup object (or vice versa): User, original object: Account: []

Trigger.SyncProductstoUser: line 65, column 1: []

Working Solution
Trigger:
trigger SyncProductstoUser on Contact (after update) 
{

    Set<Id> UserIds = new Set<Id>();
    Set<Id> ConIds = new Set<Id>();

    for (Contact contactInLoop : Trigger.new)
    {
        if(contactInLoop.Id != null && contactInLoop.Id != null)
        {    
            ConIds.add(contactInLoop.Id);    

        }
    }
    
   
        List<User> Users = [Select Id,Product__c, Product_Access__c, ContactId from User  where ContactId =: ConIds];
   
    

        Map<Id, User> UserContactMap = new Map<Id, User>();
        
        // loop through all retrieved Contacts 
        for(User u : Users)
        {
        // get all Contacts for the User
           UserContactMap.put(u.ContactId, u); // put the User Contacts in a map by Contact Id
      
        }
    



        for (Contact contactInLoop : Trigger.new)
        {
            if (contactInLoop.Id != null  && contactInLoop.Product_Access__c != null && contactInLoop.Product__c != null )
            {
                User ur = UserContactMap.get(contactInLoop.Id);
                
                if(ur != null)
                {              
                   ur.Product_Access__c = contactInLoop.Product_Access__c;
                   ur.Product__c = contactInLoop.Product__c;
                   update ur;            
                 
                }
                 
            }
        }
    
 
}

Test Class

@isTest(SeeAllData = True)

public class TestSyncProductstoUser
{
    static testMethod void UpdateContact()
    {
        Account Acc = new Account(Name='ABC CORPORATION');
        insert Acc;
        Contact Con = new Contact(AccountId=Acc.Id,FirstName='ABC',LastName='User1',Email='ABC1User1@ABC.com',Product__c='ABC', Product_Access__c='ABC User');
        insert Con;
         
         
       User userToCreate = new User();
       String ProfileId=[Select Id,Name from Profile where Name='ABC Community'].Id;
       
         
       userToCreate.FirstName = Con.FirstName;
       userToCreate.LastName  = Con.LastName;
       userToCreate.Email     = Con.Email;
       userToCreate.Username  = 'ABC@community.com';
       userToCreate.Alias     = 'ABC1';
       userToCreate.ProfileId = ProfileId;
       userToCreate.ContactId = Con.Id;
       userToCreate.Product__c = Con.Product__c;
       userToCreate.Product_Access__c = Con.Product_Access__c;     
       userToCreate.TimeZoneSidKey    = 'America/Denver';
       userToCreate.LocaleSidKey      = 'en_US';
       userToCreate.EmailEncodingKey  = 'UTF-8';
       userToCreate.LanguageLocaleKey = 'en_US';
       
       insert userToCreate;
       
       system.debug('***Contact ID' + Con.Id);
         
       System.runAs(userToCreate)
       {
           Con.Product__c = 'XYZ';
           Con.Product_Access__c = 'XYZ User';
           update Con;
       }
 
    }

}