"What is wrong with the execution of this composition"
Composition is the layout of the elements on the page (file, canvas, whatever)
The ELEMENTS are not done well - and I think there's a lot in the comments about how to fix those problems if you read and stop being all "I made a living blah blah blah". If you did, good for you. I recognize and respect that. However, I disagree that the posted artwork shows the craftsmanship of a professional.
This is sort of a TL;DR reply, just in case you feel like STILL no one has answered your question.
Doctors heal you, Artists immortalize you.
"Those who can, do. Those who can't, teach" - bullshit.
The usual staples for anatomy: