If you have a copy of your prior years return, find form 8879. Next to your authorization signature will be your 5 digit pin. You can review your previous years returns from your Tax Home by scrolling down to Your tax return and documents and selecting show.
If you are trying to file your 2020 return, this code can be any 5 digit number. I use my zip code. It does not need to be that same number each year. If you call the IRS to inquire about your return, they may ask for this 5 digit code for verification.