I'm trying single handed backhand, is it maybe lack of strenght and I should try 2 handed backhand? or should I wait a little longer before changing it?
First Id like to welcome you to the game of tennis! Then id like to tell you your coach is pretty much right but might have explained it the wrong way.
On the tour, players are hitting every ball the same way...every time. They know exactly where and when to make contact with the ball. They reach this point by practicing over and over. They at one point where in your shoes! After lots of practice your body begins to do the "right things" more consistently.
Dont get discouraged with your one handed backhand. Your wrist control is very weak if you just began playing tennis (naturally as any players would be). My suggestion is to make sure you are hitting with proper form (technique) every time you ball. With time you will have great shots, and still have your one-handed backhand!