The electronic configuration of calcium is 2,8,8,2.So why we can't write it as 2,8,9,1?

The Aufbau principle states that an electron occupies orbitals in order from lowest energy to highest.

Calcium(20) has 20 electrons.
Its electronic configuration is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^2$ instead of $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{1}4s^1$.
As the energy of 3d orbital is higher than 4s, the last two electrons in calcium get accommodated in 4s energy level instead of 3d.
It is because of this order in which the atomic orbitals are filled as stated in Afbau’s principle that the electronic configuration of calcium is 2, 8, 8, 2 neither 2, 8, 9, 1 nor 2, 8, 10.