Why can’t I file my federal return because of the retirement box?

No, you do not want to mail your return in. This is because the work that TurboTax is doing now affects how your information is reported, whether e-file or in print. Besides, print-and-mail is infinitely slower than e-file.

I am assuming that you are unable to e-file because you have an S in box 12, which concerns a SIMPLE IRA. Here is the note I have been posting:

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I suspect that you will have to wait for TurboTax to be updated. On January 8, 2024 (i.e., just over two weeks ago) the IRS released a notice describing many changes required to implement the Secure Act 2.0 - I believe that this is what "We're updating how SEP and simple plan contributions are reported on the tax return based on new guidance from the IRS." is referring to.

Indeed, Section 601 of the Secure Act 2.0 refers specifically to Simple and SEP Roth IRAs. Code S in box 12 refers to how to handle a SIMPLE plan.

These last minute IRS (and law) changes inescapably cause delays on the part of TurboTax and other tax processors. 

It will take time for TurboTax to understand all the ramifications of this IRS Notice and then code and test the changes. The IRS is not open for filing until January 29th so you have some time to wait.