During review of my MN state return, I'm getting an error on Line 2-Payers MN St ID No. I've entered the number that shows on my 1099R, but it won't accept it

Exit the review and go back to the Retirement Income section to edit the fields on your 1099-R.   Search for 1099-R and then click on Edit or Review next to the 1099-R that has the error.  You will be able to edit it from there.