When you sent the check back did you also return the tax withholding from your own money? Doesn't sound like it. Then that part will become a taxable distribution. You enter the full 1099R and say you rolled over the check amount (DVD's if you put it back into the same account). You will get credit for the withholding in box 4 on 1040 line 25b.
When you enter the 1099R be sure to say it was NOT a RMD since RMD was not required for 2020. Then it will let you say you rolled it over.
