You are also required to file for 2021 if you received advance child tax credit payments.
Did you receive unemployment benefits in 2021? If so, that is taxable income and it might be enough that you are required to file.
If you had no income in 2021 how did you support yourself? Did someone else support you? Can that person claim you as a dependent?
You can use the calculator at the following link on the IRS web site to see if you are required to file a tax return. But the first and third items that Critter-3 listed are reasons that you should file even if you are not required to.
Do I Need to File a Tax Return?
