To enter Social Security benefits reported on form RRB-1099

• Click on Federal Taxes (Personal using Home and Business)
• Click on Wages and Income (Personal Income using Home and Business)
• Click on I'll choose what I work on (if shown)
• Scroll down to Retirement Plans and Social Security
• On Social Security (SSA-1099, RRB-1099), click the start or update button

Or enter social security benefits in the Search box located in the upper right of the program screen.  Click on Jump to social security benefits

Make sure that you indicate that you are entering a RRB-1099 and not a SSA-1099