Regardless of whether or not you completed the conversion in time, you were required to report the non-deductible contribution on your 2021 tax return. That would have required including form 8606 with your 2021 tax return. If you did not report it, you need to report it now.
Form 8606 is one of the forms that can be filed by itself, because it has a signature line. You will need to download the 2021 version of the form and instructions, not the current 2022 version. Complete the form following the instructions, sign it, mail it to the IRS as indicated in the instructions, and best to use a mail service with tracking and proof of delivery. Keep a copy for your records.
https://www.irs.gov/prior-year-forms-and-instructions?find=8606&items_per_page=200
Then, you will need the numbers from the 2021 form 8606 to prepare your 2022 tax return which includes the conversion.
