I don't have an answer for you, but I thought I'd add some more details in case any other users can weigh in.  I made up a dummy return to try and replicate/solve this. I input an amount for SSI income on Screen 14.1, and I see the error you're referring to (Diagnostic 54921, I checked the diagnostic center, no solutions or workarounds are listed there).

And I agree, I don't see anywhere to input it on Screen 58.1.

You will likely need to call Support or send this return to them (Tools>Communications>Service Requests / Lacerte Email).

I don't do any 1040-NR returns, so I'm going to ask a crazy-out-there question: is it possible that you need to file a 1040 return instead of a 1040-NR because of the SSI, or that maybe it isn't reportable? Just reductio ad absurdum here.