filing

Social Security Retirement/Disability/Survivors benefits are reported on a form SSA-1099 and the benefits received are to be entered on a federal tax return. The benefits may be taxable depending on the amount of other income you have entered on your tax return.
However, if the Social Security benefits are the Only income to be reported on the tax return, then there is no reason or need to file a tax return.

If you had health insurance through the Obamacare system or you have dependents to claim for the child tax credit this year or you did not get all of the third stimulus check you're entitled to then these are  reasons you need to file a 2021 return.