Just learned an e-file was rejected many months late. How to proceed?

You can print and mail the tax return.  There are no penalties for filing a tax return after the due date if the tax return is being filed for a tax refund or there are no taxes owed.

See this TurboTax support FAQ for the procedure to print a tax return for mailing when using the online editions - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online/01/26258

What tax year?  If it was for 2018, and if you know the reason for the rejection , and it's something you can correct, then you can efile it again.   Did you use Online TurboTax?