First of all, congratulations that you want to train with a corpulent body. I can understand that it is not easy as when you are slim.
But as a beginner, if you can do some exercises but not complete the asked repetitions, it doe not mean you failed. Take a little pause, and complete the session. The most important is not to finish the session in one set, it is that you make the exercise right.
It is also very important to give the feedback to your coach right and not lying. You can also say, you want to do the exercise but you can’t ow so you need more training for it.
You also have the opportunity to tell to your coach, that you cannot do this exercise and he should not ask it again. You always can come back and select it that you want to do it:
Go to Coach
Select the sign that is on the righ all above you screen (right the trophy)
It should be written Excluded exercises (under equipment). Select it.
Surch the exercise you cannot do and select it
As I told you, you always can get there and change it when you can do the exercise.
The coach will give you another exercises you can do.
DO NOT GIVE UP. BE PATIENT. Believe to yourself.
If you can, use the app of nutrition and tells you want loose weight.
I wish you all the best!