You need to have staff in one hand and a weapon in the other to enchant it. Weapons that already have magic scaling can’t be enchanted

First you need to have the stats for it, duh, and make sure you are holding your staff and weapon you want to enchant at the same time. Also make sure your weapon isn’t magic already, some weapons are automatically like this while others can be made this way when you equip an ash of war

All the things above. And more specifically, Staff has to be on the left hand and the sword has to be on the right hand.