All I have is social security income how do I file

@Critter-3 

If Social Security is your Only income there is no need for you to file a tax return.

@DLClark 

Why are you trying to file a 2021 tax return if your only income is SSA benefits reported on a SSA-1099 ?  

@DLClark - if your only form of income is social security income AND you received your 3rd stimulus of $1400 AND there were no tax withholdings on Form SSA-1099, then there is no need to file a 2021 tax return.

AND  you did not have ACA Marketplace insurance AND  (for 2021 return only) did not have any earned income in 2019  ...   then you have no reason to file a return.