Question

Energy level diagram for Ni2+. why the electrons are taken from s orbital but not d orbital?

Answer 1

Because in this case the s-orbital you mean has a higher energy level (n=4) than the mentioned d-orbital (n=3). That means the s-orbital will loose the electrons first (the valence electrons are in n=4, so they will be the first to go way)

Answer 2

For transition metals, electrons often move from lower orbitals to higher orbitals to have have filled sub-shells.
This is also an explanation for a few of the oxidation states.
Having half filled or completely filled sub-shells make the metals more stable.