Magic missile you already HAVE _if_ you selected mage as one of your two starting character classes at character creation. Otherwise you will NEVER learn to use magic missile, regardless of buying scrolls or anything else. Unlike Guspav's older pre-2017 version, the entire quest to become a mage is removed in my remake, and has been since early 2018.

Magic missile is the spell you get if you had selected to start the game as a mage.

I should add that you will not be able to open your mage spellbook (pressing B key) in a scene or battle if you don't HAVE a spellbook in inventory. If you sold it, gave it away to a companion, or left it in storage with your chamberlain or constable, then your spell casting is down. Replacement spell books are sold at the 4 mage guilds.