When selecting a spell, players inventory reset their spells textures to a diamond axe. How can I fix this?

Does this happen if you don’t have the other resource pack installed?

Yep

If the player has /op or *, spells is normal loaded

Do you have a plugin that might be preventing players from having unbreakable items?

Yep, this is the plugin Itemfixer. Ty :)

Well that makes sense. Can you turn that feature off? All of the icons rely on items being unbreakable.

Otherwise if you are on 1.14+ you could make your own RP that does not rely on that (or wait for me to make one, which I plan to do, but not necessarily soon).

Can you turn that feature off?

Yes