The statutory limit for additions is 100% of compensation, but since the employee portion of FICA must be paid from the gross pay, that's money that is unavailable to defer to the 401(k), just as income tax withholding is unavailable to defer. The limit for deferral ends up being gross pay minus all of the withholding. The 100%-of-compensation limit comes into play when the employer contribution is factored in. Your elective deferral plus the employer contribution is not permitted to exceed 100% of compensation.
That actually suggests a simple fix as long as no employer contribution has yet been made. With $13,334 of compensation, subtracting from that the $1,450.24 of total tax withholding means that your maximum elective deferral to the 401(k) was $11,883.76. Assuming that you are the only employee, what is now a $1,450.24 excess elective deferral might be able to be re-designated as the employer contribution (and would be the maximum permissible employer contribution). Then all you would need to do is correct box 1 and the amount shown with code D in box 12 of the W-2.
Otherwise, $1,450.24 needs to be distributed and generally returned to the S corp. Unless the plan can distribute it directly to the S corp in a way that is nonreportable, this $1,450.24 would likely be reported on a Form 1099-R as taxable to you. Because it's under $3,000, if paid to you and you reimburse the S corp, you can't deduct it or claim a tax credit under a claim of right. However, instead of returning the $1,450.24 to the S corp, the S corp might be able to deduct the repayment after tax from your 2021 compensation; that would means that the $1,450.24 would appear as part of your 2021 compensation. The amount that would actually be distributed from the 401(k) would be adjusted for any investment gain or loss while in the 401(k).
