Can I add earnings to an employee's w-2 after 12/31/21 if it hasn't been filed yet?

Thank you for taking the time to post here in the Community, @stanley.7. I'm here to help you resolve your issue. 
 

You can still add a w-2 to your employee's paycheck. You just need to make sure the pay period is in 2021 so that it is correctly reported on the w-2 form.
 

To learn more about W-2s, you can check out this article. 
 

You can also refer to this article for more information about the year-end checklist for QuickBooks Online Payroll.
 

Feel free to come back if you need extra assistance with your tax forms. I'll be there to help you out. Have a good one!