For 2021 it depends. Normally if you have no income to report you don't file an income tax return.
However 2021 is unique ... even if you had no income a return is needed if :
> you have dependents under age 18 to claim and you yourself are not being claimed as a dependent
> you had health insurance thru the ACA marketplace
> you are not a dependent and you did not get the third stimulus payment
