My client left her job in 2020 and directed her tax-deductible 401k balance to be rolled over into a Roth IRA.  The 2020 1099-R had Code G and they did their own taxes in 2020 and this was reported as a regular rollover, so no taxes were paid.  However, the 2020 401k check was lost in the mail and had to be reissued in 2021, so the client didn't open her Roth until March of 2021 when she received the check.  

To record the tax on the Roth conversion, do I amend the 2020 tax return to include the Roth conversion or is there a way record it on the 2021 return, since that's when she actually received the payment.