I believe the solution has been found, or at least it worked for me on a different tax app and will probably work on TurboTax, too, and the problem was not the five digit PIN.
As we all know, when filing jointly, the app does not ask for our Prior Year AGI individually. In a joint return, we have one Prior Year AGI and that's the way it's entered. Apparently, the problem is a glitch in the app for joint filers, probably in the IRS system, that recognizes the Prior Year AGI as belonging to the tax payers but not the spouse. Here is the solution that worked in FreeTaxUSA and will probably work in TurboTax:
1. Go to the page that asks for your Prior Year AGI.
2. When asked if you filed a 2020 tax return, answer yes.
3. Even if you filed jointly in the past, click on the option that "Only the taxpayer filed a return".
4. On the Prior Year AGI or PIN page answer YES, that you have access to the primary taxpayers AGI.
5. On the Prior Year AGI page, enter the correct amount of your joint return's Prior Year AGI as if it's for the primary taxpayer only. This will make the app assume that the spouse's Prior Year AGI is 0.
6. Continue and ignore any alerts and fill out the rest of the pages until you can press the Send Tax Return button.
