This seems to suggest that you previously reported the rollover as a rollover to a traditional IRA instead of to a Roth IRA, causing the entire box 2a amount to be excluded from line 5b of your originally filed Form 1040. If the amount originally shown on line 5b is does not include the entire amount from box 2a of your original 2020 Form 1099-R, this is likely what happened, meaning that your originally filed tax return underreported income.
The 3-year statute of limitations on filing applies to a claim for refund. Your amendment will not be claiming a refund.
