Hello I hope you could help me to solve my problem there you have it when my laceur reaches the apoapsis MechJeb does not want to finish the orbit and my launcher falls down so I hope you could help me the version of the mod is 2.12.3.0

You probably don't have enough torque. Add more reaction wheels and try again

Sometimes it helps to switch the attitude algorithm between "MJAttitudeController", "HybridController" and "BetterController" which depends the vessel mass and the scenario.
Just open the corresponding UI in flight and switch it to your needs.

There is no single algorithm capacle of everything, imagine it as being like this:
YOU aka. your brain is the main algorithm, you just USE the MJ algorithms as extension fitting the scenario.