"He is in a variety of fields."
Translation:Él está en una variedad de campos.
I guessed the phrase could do without the Él at the front. But apparently that was wrong.
Duo is pretty sloppy in its translations this late in the game (paucity of contributors?) You may not be wrong. Shall we report it?
This is not a location, it is more like a profession or teaching / research subject issue, ie what fields do you work in. And in that case, shouldn't ser and materia be acceptable.