The Form 5329 that needs to be filed to request a waiver of the 50% excess-accumulation penalty is a 2021 Form 5329, filed with your 2021 tax return.
Note that if 2021 was the year that you reached age 72, the deadline to receive the 2021 RMD is April 1, 2022 and taking this distribution by then would mean that there was no excess accumulation for 2021 that would need to be reported.
