Thank you for the reassurance. I read around on other posts and it seems it is indeed not supposed to be possible to file the amended return with the online version for the 2021 tax year. The error/rejection message that results is just how TurboTax's system is responding to this glitchy sort of interaction.
To clarify for future readers, this rejection is on the TurboTax side and is not a rejection from the IRS. Simply print and mail the amended return you created before. There isn't actually any "data missing or invalid" in the return necessarily.
I appreciate all commenters' help.
