Steps to reproduce:
1. Turn npc-editor-op-only to false in pixelmon's config.yml
2. Set your gamemode to survival and give yourself the editor.
3. Try to use it, it will display "You must be an OP or have permission to perform this action."
When that config option is false, pixelmon only checks if you're in creative or not. You can be opped during this, and it will still display that message. Either way that config option is set, the NPC Editor does not work in survival mode. If that config option is true, when in survival, it displays no message when you try to use it. I believe this is what it should also do.