Transition metals are those elements which have incomplete D orbital in ground state or common oxidation states.

Cu- [Ar] 4s1 3d10

Cu has d10 configuration in ground state but in its most common oxidation state i.e Cu+2 it has d9 configuration Hence it is a d block element and a transition element.

Then,  If Zn is transition metal or not?

Zn- [Ar] 4s2 3d10

It is not as in ground state it has d10 configuration and in most common oxidation state i.e Zn+2 it also has d10 configuration. Hence Zn is d block element but not transition. (Same for Cd,Hg)

아 그리고 nickel(2) chloride 는 green colour 인데 그럼 red light 를 흡수했다는 거잖아요 그럼 longer wave length 를 흡수했다는 뜻인 건가요?

YEs, it absorbs the red light and tranmiits the green light.  Red light has longer wavelength than the green light.