Sc (Z=21) is a transition element but  Zn(Z=30) is not because 

Sc (Z=21) has incompletely filled 3d-orbitals in its ground state $\left(3d^1\right)$, it is considered as a transition element but Zn(Z=30) has completely filled d -orbitals $\left(3d^{10}\right)$ in its ground state and its common oxidation state of (+2), it is not considered as a transition element.