eFile is not available for prior year returns. You must print and mail the old-fashioned way (or USPS Priority Mail, if you're worried about timing). No way around it. The actual window to eFile for prior years is closed. If you're referring to your 2020 return, you should still be able to eFile... I just did today...so, I don't know why it's not available to you. Good luck.
You do not say why e-filing is not available to you---what do you mean? Is your e-file being rejected?
Some rejected returns can be fixed and re-filed. Some have “un-fixable” reasons for the rejection and have to be mailed instead. What is the rejection code or rejection message?
If it was rejected for AGI—-be aware that many 2019 returns took so long for the IRS to process that your 2019 AGI might not be in the IRS system. Try using zero instead.
If filing with zero is rejected then file again but select the option that you did not file last year. The IRS does not see that question and it will allow you to e-file with no AGI question.
Or—-if you filed as a “non-filer” just to get the stimulus check on a 2019 return, try using $1 as the AGI.
If that does not work and you still cannot e-file, then print, sign, and mail your tax return.
If you want to just mail your return you can print it to mail. Be aware that processing mailed returns will take longer--the IRS is backlogged so it will take several months for them to process your mailed return.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.
Federal and state returns must be in separate envelopes and they are mailed to different addresses. Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.