Which statement is incorrect?

In $Ni{\left(CO\right)}_4$, Ni has zero oxidation state. It is $s{p}^3$ hybridised.

Hence, it has no unpaired electron so, it shows the property of diamagnetism and has tetrahedral structure.
NOTE
The valence shell electronic configuration of ground state Ni atom is $3d^{8}4s^2$. The two electrons of 4s are pushed into 3d orbitals and get paired up because of the presentce of strong ligand (CO).