I filed my taxes for 2021. After I received my refund, I realized I missed a 1099R for my wife. The taxable amount is zero but there is a gross distribution.

An amendment is not required if there is no change in your tax liability. 

Eventually the IRS will send a CP2000 notice for the missing income so filing an amended return now to get ahead of the curve is allowed or keep your records so you have the ability to address this in the future.   If you efiled the original return and you have already gotten the original refund or paid the original balance due then you can efile the amendment thru the TT program until 10/15/22.