945 withholding for 2021

Accountant paid the withholding for a participant in 2022, but payment was for a 2021 distribution.

How to handle?  Client knows there will be penalties

That happens.  When paid in EFTPS, the payment is to be marked for tax year 2021, even though paid in 2022.  That's fine as long as it meets that particular entity's deposit deadline.