Honestly I would consider this a UI/UX issue rather than a localization issue. The problem isn't that the message is bad (it is bad, but that's not the problem). The problem is that the system probably isn't coded to put out more specific error codes that need to be translated to human readable messages. The system should have different error codes for
  • Equip level too high
  • Wrong race
  • Wrong gender
  • Wrong class/job
  • Multi-slot conflict

and each of those should have a localized message so that it's clear what the problem is and less experienced folks aren't left just scratching their heads.