Yes, if you can't fix the reject error, it's likely best to send in a paper return. Electronic (eFile) return validity checking software is sometimes different from the validity checking IRS performs on paper returns. Paper processing is slower, so if you're due a refund it may be slightly delayed, but frequently the error causing the eFile reject can be either bypassed or corrected in the upfront IRS input processing.
However, if you're also implying that your Tax Year 2019 return remains unprocessed, although that fact shouldn't directly prevent your 2020 return filing, there may be a similar error on both returns which is preventing a successful filing.
