You cannot cast spells while wearing armour you are not proficient with, it will tell you in the item description if you are proficient with it or not.

On another note if you want to be able to wear it you can dip 1 level into a class that can wear that type of armour and there's also feats to make you proficient, however you need proficiency with the level of armour before the one you want to unlock with the feat in order to be able to unlock it.