During 2020 some employees may have elected to defer social security taxes. When that tax is repaid in 2021 you will need to make a report to the IRS. According to this Sage KB article which highlights the IRS Form W-2 Reporting of Employee Social Security Tax-Deferred under Notice 2020-65.

According to the IRS:

On August 8, 2020, a Presidential Memorandum was issued, directing the Secretary of the Treasury to use his authority pursuant to section 7508A of the Internal Revenue Code to defer the withholding, deposit, and payment of certain payroll tax obligations. In response to the Presidential Memorandum, Treasury and the IRS issued Notice 2020-65 on August 28, 2020. Notice 2020-65 provides employers with the option to defer the employee portion of Social Security tax from September 1, 2020 through December 31, 2020, for employees who earn less than $4,000 per bi-weekly pay period (or the equivalent threshold amount with respect to other pay periods) on a pay period-by-pay period basis.

The employee portion of Social Security tax deferred in 2020 under Notice 2020-65, as modified by Notice 2021-11, that is withheld in 2021 and not reported on the 2020 Form W-2 should be reported in box 4 (Social security tax withheld) on Form W-2c, Corrected Wage and Tax Statement. On Form W-2c, employers should enter tax year 2020 in box c and adjust the amount previously reported in box 4 (Social security tax withheld) of the Form W-2 to include the deferred amounts that were withheld in 2021. All Forms W-2c should be filed with SSA, along with Form W-3c, Transmittal of Corrected Wage and Tax Statements, as soon as possible after you have finished withholding the deferred amounts. See the 2021 General Instructions for Forms W-2 and W-3 for more information about completing and filing Forms W-2c and Forms W-3c. Forms W-2c should also be furnished to employees, and you may direct your employees to (or otherwise provide to them) the Instructions for Employees, below, for instructions specific to this correction.

Please consult with your financial advisor prior to making any filings to ensure that your tax situation applies to this specific IRS notice.