I am wondering if it is possible to make it so a subclasses ac is something along the lines of unarmed defence. For example I would like to give my warlock subclass an AC ability which is 12+charisma modifier. Is this possible and if so how would I go about doing this?

There are two steps to doing this.

First, add the modifier: type:set / subtype:unarmored armor class / ability score:CHA / fixed value:2

Second, add the modifier: type:ignore / subtype:unarmored dex AC bonus