Hi,
I have one Custom object and with lookup of User object, When we save this custom object first it will check the related user is active or not. if not active then it has to activate the user and save record.
One @Future method is allready written for User object.

Here is the error which i am getting:
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: ClientReview__c: []

Here is my method which i wrote in class:
 Public void ToActiveateUser(List<ClientReview__c> clientReviewList)
    {
      List<ClientReview__c> alertReviewList = new List<ClientReview__c>();
        Map<String,RepCode__c> repCodeMap=this.getRepCodeMap(clientReviewList);
        List<Id> AdvisorUsers = new List<Id>();
        for(ClientReview__c review:clientReviewList){
            String repCode=review.ClientRepCode__c;
            if(repCode !='')
            {
                RepCode__c rc=repCodeMap.get(repCode);
                if(rc!=null)
                {
                    String ownerId=rc.Advisor__r.Advisor_User__pc;
                    if(ownerId != '')
                        AdvisorUsers.add(review.OwnerId);
                }   
            }
        }
      if(AdvisorUsers.size()>0)
      {
         List<User> advisorUser = [Select Id,IsActive FROM User WHERE Id IN:AdvisorUsers];
         List<User> updateuser = new List<user>();
         for( User usr: advisorUser)
         {
           if(usr.IsActive == false)
           {
               usr.IsActive = true;
               updateuser.add(usr);
            }   
         }
         if(updateuser.size()>0)
            update updateuser; 
             
      }  
    }

And i am calling this method in Trigger:

Trigger Supervision_Sharing on ClientReview__c (after insert) {
    SupervisionOperations so=new SupervisionOperations();   
    so.ToActiveateUser(Trigger.New);
}