Lots of useful info here to get you started.
It appears the exercise is too hard for your client or their form needs some work.
I’d start with the reason why you prescribed the exercise then regress from the push up backwards to a resistance band.
Test your clients shoulder stability, ask about wrist conditions, think about their body composition and experience then prescribe an alternative.
It’s important for the clients self efficacy to successfully perform the exercises we give them.
They tend not to come back if they don’t feel good because they found the workout too challenging.