Can Mark’s not claim credit for his 401K contribution before taxes.

[Opus 17]

401k contributions can only be made by payroll deduction and they are already subtracted from your box 1 taxable wages on your W-2.  You can't take a separate deduction because your income was already reduced. 

[MayaD]

Yes, you don't have to claim the Retirement Savings Contributions Savers Credit

To remove the credit :

• Login to your account.
• Select Federal from the left menu.
• Go to Deductions and Credits 
• Scroll down to Retirement savings contribution credit in the Retirement and Investments section
• Continue to the screen Retirement savings contribution credit and answer Yes to the question "Were you a full time student during any five month in 2022?" ( even if you were not a full time student)

You won't qualify for the credit if you check that you were a full time student.