I get the same result. Looks like a bug.
Very interested in hearing the resolution. Thank you for bringing this up.
The second argument to UnitConvert has to be just a unit specification. This works
w = EntityValue[Entity["Country", "UnitedStates"], EntityProperty["Country", "FreshwaterWithdrawals"]];
UnitConvert[w, "Giga" "Feet"^3/"Years"]
That is good news. But why would an entity return an invalid unevaluated UnitConvert?
I tried the code I posted on 12.1.1 and now it fails. I tried 12.0 and 12.1 and sometimes the first evaluation works but subsequent evaluations fail. Seems like a caching bug in the entity framework. To get it to reliably work on the first evaluation
but it will fail on subsequent evaluations.
@Luc Barthelet You should report this to WRI.
I filed a bug .[CASE:4563074]
Good luck team!