should it be in instrumental: "Jestem niepełnosprawnym"
It depends on context, if you treat "niepełnosprawna/y" as adjective or noun. It is an adjective and adjective without noun is in nominative after "jest/jestem..."
But if you treat it as noun (I am a/the disabled) then you use instrumental.
To avoid cross posting, I've moved my (since expanded) dissection of nie-pełno-sprawn-a|y to the fuller thread at https://forum.duolingo.com/comment/19515272 .
[10 Feb 2019 12:24 UTC]