I just filed two automatic extensions online, one for me and one for my son. One was accepted and one rejected . I can not tell whose extension was rejected!

the response said:
"The email you received will include the specific reason why your extension was rejected:

Select the orange Fix my Extension link in your email.
Sign in to TurboTax Online.
Once you've signed in, select Fix my extension."
BUT, there is no Fix My Extension link

Did you use separate online accounts?  If you used the same account it would probably have rejected the second one.

You can check out this FAQ on how to fix a rejected return and it should have the information on which one it was. https://ttlc.intuit.com/replies/4552370