I just got a PS5 on Monday, so I'm new to the next-gen version of 2K21. On the PS4 version, I was having the opposite problem as you. I was scoring in the paint at will. In fact, it was the only way I could score besides free throws because shooting was so difficult.
On the PS5 version, I was initially having the same problem as you. I play on Superstar. In one of my first games, I only scored 20 points in the paint, which is the lowest PIP total I ever had in playing 2K or any basketball game, and I've literally played every 5-on-5 basketball game dating back to Double Dribble on the NES.
But since yesterday, I'm now scoring in the paint as consistently as I was on the PS4. For me, the key was fully immersing myself and attacking the basket using real basketball principles rather than playing it like a video game. I read the defense and think ahead how everything will play out once I make a move to the rim.
"What defenders are in position to react to me?"
"Any shot blockers I need to worry about?"
"If I can't get a clean look, what is the likelihood of at least drawing a foul?"
"Does the player I'm controller have a quicker first step than the defense?"
Also, here are some videos I watched back in the day whose principles can be applied in 2K21:
How to Finish Through Contact:
https://www.youtube.com/watch?v=lytUfw3-LLw
How the BEST Small Guards Finish at the Rim:
https://www.youtube.com/watch?v=66sGWSoDJbk