I was trying to setup command blocks so that the players on my server could automatically change the size of their pokemon.
I setup a command block for every size with the following command:
/pokeedit @p 1 gr:ginormous

This way they could change the size of the pokemon in their first slot. If they wanted to change the size of a pokemon in a different slot they would have to swap it to the first slot since I am not going to place 48 command blocks.
But this command gave me the following output:
This player doesn't exist.

I tried the /pokeevolve command after and that one gave me the following output:
You do not have permission to use this command.

I thought commands in a command block were always executed by the server. Therefore does that mean the server does not have the rights to execute commands?

Extra mods installed:
BetterFPS, Chameleon, Chisel, CraftTweaker, CTM, CustomMainMenu, DefaultOptions, FasterLadderClimbing, FastFurnace, FastLeafDecay, FastWorkbench, FTBBackups, FTBLib, FTBUtilities, InventoryTweaks, IronBackpacks, IronChest, JEI, JEIIntegration, JourneyMap, Levelup2, ModTweaker, Morpheus, MouseTweaks, MTLib, NetherPortalFix, OreExcavation, PixelExtras, Pixelmon, PixelmonInformation, Placebo, Reauth, StorageDrawers, StorageDrawersExtras, Theoneprobe, Toast Control

Hey, actually i think its good idea
i'll try to see if i can add this to PixelExtras, and if mod dev are okay with my commit.
In other words, if everything is good, next version should support the @p & @r !

This has been fixed in an upcoming version, thanks for the report!

-Craft