If you contributed to a 401K through your employer, then that information is likely on your W2 in box 12 code D.
Retirement Savings Contribution Credit is a credit for making eligible contributions to your IRA or employer-sponsored retirement plan (i.e 401k). It is designed to encourage those with adjust gross income levels below a certain amount to save for retirement. You can find more information about these levels at the link below. If you did contribute to a retirement plan, then you should click on that credit and go through the questions to see if you qualify. The program will pull the information from your W2 into that section automatically, but it will ask you some follow up questions and also if you made contributions to another account like an IRA.