If a level 20 Wizard (School of Illusion) casts Invisibility on a creature and then uses its 14th level ability illusory reality to make the invisibility "real", what happens?
Illusory Reality says (PHB, pg. 118):
you can choose one inanimate, nonmagical object that is part of the illusion and make that object real.
A creature is not inanimate, and invisibility is not an object.