There are very few elements which are not supported - Pr is one of them.
The full list is: Pr, Nd, Pm, Sm, Eu, Gd, Ac, Pr, U, Np, Pu.
We don't have any good quality pseudopotential and/or basis set for these elements right now. In general the rare earths require special methods, since they have f-electrons in the valence, and strong spin-orbit interaction.
Note that La is supported as of 11.8, but not in earlier versions.