Did you prepare it and just forget to file it? Or did you forget to prepare it/file it?
If you prepared it, then the only way to file it is to print, sign and mail it now.
If you did not prepare it, then you need the desktop (CD/download) software on a full PC or Mac to prepare it, then you can print, sign and mail it in.
https://ttlc.intuit.com/questions/1900990-how-do-i-file-a-prior-year-tax-return
Or....you can get the forms from your state and prepare them by hand to mail in. You will need information from your 2020 federal return.
https://ttlc.intuit.com/community/state-taxes/help/how-do-i-contact-my-state-department-of-revenue/00/25573