“Box 12 code-We're updating how SEP and simple plan contributions are reported on the tax return based on new guidance from the IRS. Please check back with us soon.”

I suspect that you will have to wait for TurboTax to be updated. On January 8, 2024 (i.e., just over a week 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.

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.

Please check back on or after January 31, 2024, which is when we tentatively anticipate all of the necessary work will be completed. 

Thank you! I had this issue today and couldn't find any reference to pop up for guidance.

Any update to this issue?

Please try to file again; many taxpayers have reported success in doing so.