names=# INSERT INTO public."Usercomp"("Shiftstart")
names-# SELECT "Shiftstarttime"
names-# FROM public."Shifts"
names-# WHERE "Id" = public."Usercomp"("Shiftid");

ERROR:  column "Shiftid" does not exist
LINE 4: WHERE "Id" = public."Usercomp"("Shiftid");
                                       ^
HINT:  There is a column named "Shiftid" in table "Usercomp", but it cannot be referenced from this part of the query.