Did you report the 1099R distribution at all on your 2020 return? Is it on 1040 line 4a and how much on 4b? You might have reported 100% in 2020, in that case you are done and don't need 8915 to split the income over 3 years.
8915-E is only for tax filers who were affected by Corona Virus in tax year 2020.
If you were affected and your age is below 59 1/2, you paid a penalty for early withdrawal that is waived.
You'd have to amend.