Are letter in mail rejected return "invalid account number structure"?

Letter from whom?   Bank account? Is it a US one?

It is from the IRS. No money has been taken from the account, so that checks out, but seeing as I owe an entire year's worth of taxes, I'd love to confirm where the issue was on my Turbotax return before I submit the sum online in the IRS portal. So my question is, where can i see the account details in the e-file process from turbotax?

Open your TT return and go to the File tab.

I don't see a file tab... My return was submitted and I got emails that state and fed were accepted so that might be from the pre-file view?

You can go though the FILE tab again to see what you put down.
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Some banks use different routing numbers for Deposits and Debits.  Especially Bank of America has 5 different numbers.  You might want to confirm with the bank or credit union which account and routing numbers to use.  

Click on Add a State to let you back into your return.

R04 - Invalid Account Number
The account number structure is not valid. The entry may fail the check digit validation, or may contain an incorrect number of digits.
<a rel="nofollow" target="_blank" href="https://en.wikipedia.org/wiki/Check_digit">https://en.wikipedia.org/wiki/Check_digit</a>

Thank you. I was able to see that the last digit was missing, and pay the amount online on the IRS portal. I have not received any notice about the State tax, but I assume it will have a problem since both were filed through TT at the same time.I  would love to pay that one as well before any fees are incurred. Does anyone know where to pay / check on that? I see links for refunds but not payment receipts...

Go to your state's website.  They probably have a link to pay.

Then Check the printout or PDF of your return; look for the state cover sheet with the Turbotax logo.  If you owed tax, it will show the payment information and how/when you decided to pay.  Read the state payment instructions carefully since most states cannot be paid from within the TT system and requires additional steps.
https://ttlc.intuit.com/questions/1900884-what-are-my-options-for-getting-my-state-refund-or-for-paying-any-state-taxes-i-owe