An ‘f’ block element without any f electron is

The expected electronic configuration of Th is [Rn]5f¹ 6d¹ 7s²

however, the observed electronic configuration of Th is [Rn]5f⁰6d²7s²

So, Th has no electron in 5f orbital and 2 electrons in the 6d orbital because empty,half filled,and completely filled f orbital is more stable than any other configuration . Thus, Th acquires this configuration to increase its stability, however it is a member of Actinide series