You can take 2 signed copies of the return to a local tax office and they will accept one for filing and mark the other as accepted for processing ... that should work for the mortgage folks OTHERWISE you will need to find a local tax pro that can still efile the 2020 return and pay them to do so.
Give your loan officer a copy of 2021 Form 1040.
If they want 2020 also, print three copies one for you, the IRS and the bank.
For the bank, Form 1040 Page 1-2 is usually all they want.
They usually want you to sign the copy.