Copper atom has completely filled d-orbitals in its ground state but it is a transition element. Why?

A transition element should contain incomplete d-orbital to be considered as one. The atomic number of Cu is Z=29, ideally it should have an electronic configuration of 4s² 3d⁹. This ground state is hypothetical and is not observed in Cu, due to a more stable available configuration, is the electron from s-orbital is put in d-orbital it forms a completely filled which we know is stable. So, it attains a 3d¹⁰ configuration.

The orbital diagram can be considered as follows:

Electronic configuration = 4s¹ 3d¹⁰

Ground state

So according to the definition of transition elements Cu²⁺ has an empty d-orbital and thus can exhibit the properties of a transition element.