My tax return was accepted in March 2021 but due to me needing to fill out form 8962 it's still processing. When can I amend it due to forgetting W2s?

[Critter-3]

Wait until you have the original refund in hand before starting the amendment process.   Then you MUST make the changes the IRS did to get the same refund amount.   ONLY THEN can you start the amendment process.