Initially I was trying to set up a Double Battle with an NPC trainer. First, I set the battle rules to Double. Then I get the UUID by clicking the npc with the quest editior, and then use the command /pokebattle @p <UUID> to try and start a battle. This results in "Participant 2 is invalid. Please try again.".

On the discord, it was suggested that the game may be trying to force a single battle in place of double. In response I created a new NPC for a single battle and went through the same process to start a single battle as a test. This also results in the same error. I also tried using @e[type=npc_trainer,distance=..10] and the same error message will show each time.

I have also tried initiating a double battle between three participants, with no success(same message).

While I was testing, the npc was unable to be right clicked with the npc editor after attempting the command. Left click to remove still worked. I created a new NPC each time because of this.

It was then suggested to use the NPC's name in place of the selector, and this worked! However, the bug is still persistent in the most recent pixelmon update where the backspace key is not able to be pressed when renaming Pixelmon NPC Trainers. Because of that other issue, using the npcs name is not a very good substitute because there are only so many default npc trainer names.

Greetings,

This is more of a suggestion on the current command than a bug.

The command looks for Player or Trainer names. Should you make a datapack with some unique names and assign them to an NPC in the area, the command does work as intended.