OK I finally got through this. The error is emitted when the PR's ID number is in the SS format: [social security number removed]. T-Tax thinks that this is an individual so the second designated individual isn't needed.
Changing the PR's format to the (EIN?) format: 12-3456789 clears the error and allows you to fill in the designated individual.
I changed the format in the K1 worksheet. I didn't have to uncheck/check the PR box, just reformatted the PR's ID.
If you change the format in the step by step "partner information" this will also work.
It looks like t-tax formats these IDs as SSN if you just type it in without any dashes; that was the root of it all for me. If you start off with xx- it enters correctly.
