You have 3 years to file.  You can efile until Oct 15.   If you are getting a refund there is no penalty for filing late.   If you have a tax due the payment was due on May 17 (for 2020 returns).   If you pay after May 17 you will owe interest.

If you did not get an extension and you owe the IRS money, they will charge you a penalty and interest for being late.  If the IRS owes you money, they don't care if it is late.  It is just that much longer that they can keep the money and not have to give it to you.