HI

This is the code, You can refer, You need to change your trigger little bit.

trigger Trigger_ValidateGrandTotal on Account (before update) {
    public decimal GrandtotalLimit=0;
    List<Admin__c > adminvar = [select Name,Grand_Total_Limit__c from Admin__c  where Name='CallPlan' and ownerId= :UserInfo.getUserId()];
    if(adminvar !=null && adminvar.size()>0){
        GrandtotalLimit = adminvar.get(0).Grand_Total_Limit__c;
        for( Account acc:trigger.new){
             if(acc.Grand_Total__c > GrandtotalLimit){
                  acc.addError('You have exceeded the limit of '+GrandtotalLimit+' for Account '+acc.name+'. Please review and modify the allocation');
                  break;
                    
              }                
         }
    }     
 }

@isTest(SeeAllData = true)
private class AccountTriggerTestClass {
    static testMethod void validateHelloWorld() {
        Admin__c b = new Admin__c(Name='Behind the Cloud', Grand_Total_Limit__c = 12.00);
        insert b;
        User userId = [SELECT id FROM USER WHERE id =:UserInfo.getUserId()];
        b.OwnerId = userId.id;
        upsert b;
        Account act = new Account(name = 'testaccount',Grand_Total__c = 7.00);
        insert act;
        act.Grand_Total__c = 20.00;
        upsert act;
    }  
}     

 If this post answers your questions, please mark it as solved and give kudos if this helps you

  Thanks