"from claiming the contributions in 2021 to the same plan that the 2020 8915 distribution was taken from is paying the distribution back?"

If they were submitted as contributions, they are not also repayment. For instance, paycheck deduction for 401(k), or just now making a contribution to the IRA, but also trying to take it as deductible or not. Details matter.

"to be able to enter the repayment amount"

You have to read the instructions. We don't know anything about this person or their account type or how they filed the distribution treatment in 2020.

The two questions you have will include too many variables for people on the internet to answer. You have to look it up for yourself, and apply what is specific for that taxpayer's situation.