Why does Calcium have a higher ionization energy than Aluminium?
Ionization energy is the minimum amount of energy needed to remove most loosely bound electron (valence electron).
The atomic number of Calcium  is 20. So it's orbital configuration is $\mathrm{1s^2 2s^2 2p^6 3s$2 3p^6 4s^2}$. This is the arrangement of electrons in the shells. No free electron is present in the above configuration, all the electrons are bonded. So for removal of an electron from the outermost shell higher amount of energy is required.
In case of Aluminium, the atomic number is 13. It's orbital configuration is $\mathrm{1s^2 2s^2 2p^6 3s^2 3p^1}$. One single electron is present unbonded in outermost shell, so it can be easily removed as compared to Calcium.
So, ionization energy of Calcium is higher than the ionization energy of Aluminium.