I have a new client for 2021.

2019 return had Form 8801 with carryover from prior year partially used, balance was to be carried forward to 2020. Tax preparer of 2020 did not include it on tax return. There was no 8801 on 2020 return, and no 6251 on 2020.

Do I have to amend 2020 to claim the prior year credit carryover? 

Or can I just input the amount on 2021 Form 8801 Line 19 even tho it says 2020 credit carryforward?