I forgot to add my HSA to my tax return and it's already on its way. Will my return be rejected due to that?

[xmasbaby0]

Maybe.  Maybe not.  But right now your return is in pending so you just have to wait and see.

MY RETURN IS IN PENDING—what does that mean?

Did you e-file?  Did you click that big orange button that said “Transmit my returns now?”  If so, your return is now in “pending” on a server.  Do not try to change anything while your return is in pending.On January  23 the IRS will begin to accept/reject e-files.   Watch your emails—including your spam folder—for the notice about whether your e-file has been accepted or rejected.  If your return is rejected you will be able to make corrections before re-filing.   If the return you e-filed is accepted you will have to wait until it has been fully processed and then amend it to fix any mistakes or omissions.  It has been taking the IRS six to nine months—sometimes longer—to process amended returns.   If you e-filed your state return there will be another email later that tells you if the state was accepted or rejected.

WARNING:   Do not try to change anything on your return while it is pending.

https://www.irs.gov/newsroom/irs-sets-january-23-as-official-start-to-2023-tax-filing-season-more-he...

[Opus 17]

Probably not. The IRS does not match at that detailed level when accepting tax returns into their system for processing.  It’s possible that processing could be delayed and your refund held up until you clear up the discrepancy. However, the most likely outcome is that your return will be processed as submitted, and then you will have to file an amended return to correct the issue.  Don’t try to file an amended return until after your original return is fully processed and any refund that you claimed is paid.