The magnetic moment (spin only) of $[{\text{NiCl}}_4{]}^{2-}$ is:

In $[{\text{NiCl}}_4{]}^{2-}$, Ni is in $+2$ oxidation state.${\text{Ni}}^{2+}=3d^8$.${\text{Cl}}^{-}$ is a weak field ligand, so pairing does not occur.Electronic configuration: $t_{2g}^6{e}_g^2$ (in tetrahedral field, it's $e^4{t}_2^4$).Number of unpaired electrons $(n)=2$.Magnetic moment $\mu =\sqrt{n(n+2)}=\sqrt{2(4)}=\sqrt{8}\approx 2.83\text{BM}$.