Why isn't the scandium(2+) cation isoelectronic with potassium?
The neutral $\ce{Sc}$ atom has the electron configuration of $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 3d^1 4s^2}$, and the $\ce{Sc^2+}$ ion has electron configuration $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 3d^1}$.
The neutral potassium atom has the electron configuration of $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 4s^1}$. While both $\ce{Sc^2+}$ and $\ce{K}$ have the same number of electrons, their electron configurations are different and therefore they are not isoelectronic to one another. (Isoelectronic implies same number of electrons as well as the same structure)