I'm trying to figure this out, but essentially all I'm trying to do is make a detrimental stamina aimed spell, and no matter what I do the NPC I made it for won't cast it. Also, if I try to give myself the spell, the game acts like it doesn't exist.
Im trying player.addspell but it just says that the item couldn't be found

Does anyone know of some common mistakes that would cause this sort of thing?

I made a tome for the spell, and it seems that I can cast it just fine. My NPCs simply refuse to.

I think I know what's going on. As soon as I made a damaging spell, the NPCs seem to have no trouble using it. I don't think that NPCs will use aimed spells that do not damage the target however. Unfortunately I think I will have to add small damage components to many of my spells.

EDIT: Yep, that worked.

The FormID's first 2 digits are from load order you need to find the value for this by checking what index that esp is loaded as.

NPC's generally(as in when not forced by script) will only cast Restore Health, Shields and Wards on themselves and will cast Destruction, Conjuration and a couple Illusion Based offensive spells, Scripted spells will never be cast by NPC's unless you also give it one of these other effects or force the npc to cast it with scripting.

NOTE: NPC's will sometimes casts other spell types but its generally very rare I have NPC Mages I gave nothing but spells that did not conform to these guidelines and they end up being Melee Mages in game when i disable their scripting, and almost never cast any of the 20 odd spell options I gave them for testing.

From what I've learn, NPCs wont use any spells that requires 0 magicka (FYI: Why NPCs will NEVER use their racial powers) And they will only use Desturction and Conjuration (NPCs will use a Bound Spell if they have no weapons; Falmers do all the time) fully. They will also use any Shield spells(Flesh and Ward) and Self-Healing Spells. That is about it.