Manganese(VII) oxidation state A transition element is one which forms one or more stable ions which have partially filled d-orbitals. Manganese (Mn) could have an oxidation state of 7+, which means that it will lose all of the $\ce{3d^5}$ and $\ce{4s^2}$ electrons. So how could it make a 7+ ion in that case (which means that it will lose all of the $\ce{4s^2}$ and $\ce{3d^5}$ electrons) and still be defined as a transition element?
The fact that manganese has a stable oxidation state in which it has (formally) lost all its $d$ electrons does not mean that it cannot be considered a transition metal, because it still has a lot of other oxidation states (like +II in $\ce{Mn^2+}$ or +VI in $\ce{MnO4^2-}$) with partially filled $d$ orbitals. Moreover, a naked $\ce{Mn^7+}$ ion does not exist in solution, it is only stable in the form of the oxocomplex $\ce{MnO4-}$.