I am new to UE4 and having this problem on creating ai car
I need a solution please.
There is not enough information here. So, im assuming your referencing an Ai character in your level blueprint? Did you tell your Ai Controller to run a behavior tree on its begin play? Did you give your Ai your own custom controller? Did you assign it to your Ai controller? And most likely your problem is that your doing this in the level blueprint which is almost never a good idea.
Awesome, I’m glad you got it working 
Could you please mark the solution as correct so that others with a simular question can easily find this? Thanks!
Thanks man. I forgot to asign my ai controller. Problem solved.