There is no way for you to correct the error. The employer has to correct it. They can still make the correction, even though you no longer work there. The page at the following link on the IRS web site outlines the procedure for the employer. The IRS page has a link to Revenue Ruling 2009-39, which gives the details of the procedure for the employer.
Correcting Employment Taxes
There would be no reason for the employer to issue a W-2c for 2021, because they have not yet issued your W-2 for 2021. The W-2 for 2021 that they send you next January should show the correct amount of Social Security and Medicare taxes. All you will have to do is enter the W-2 as-is in your tax return.
Direct Pay on the IRS web site is for paying income tax, not FICA taxes. You cannot pay FICA taxes with Direct Pay.
