I received a letter from the IRS stating I did not file my 2020 state taxes. I have used TurboTax for over 5 years. How can I file 2020 state tax?

The IRS is concerned with federal tax, not state tax.  Did the state send you a letter?  And does it say that you did not file a 2020 tax return at all ---- or does it say you did not pay your 2020 tax due?   TurboTax never knows if you paid your tax due.  Your only proof that you paid would be your own credit card or bank statement.

Re-read your letter and make sure you understand what it is saying.

AUDIT SUPPORT

TurboTax has a special phone number for help with IRS/state tax letters, which will be easier than going through regular Customer Support.   At the page below, click on the blue button that says "Get Help from TurboTax Support".  Then it will ask you what tax year is your letter.  Then it will ask you what the letter concerns. Then it will show you the phone number during posted business hours.

https://support.turbotax.intuit.com/irs-notice/audit-support/