Bukkit

What steps will reproduce the problem?
1. Adding a CommandSign to a Pressure Plate
2. Player is walking over the Pressure Plate and is executing the set command

What is the expected output? What do you see instead?
It works but: (see at errorlog)

What version of the product are you using?
R: v1.9.3.1 for CB 1.7.9-R0.1 Jul 15, 2014

Do you have an error log of what happened?
http://pastebin.com/SRgFmfeU

What command did you actually try to run and what did you put on the command sign (exactly as you put it). Without this its pretty hard to tell what you did incorrectly, though I would guess, as usually the issue remains the same, that the command must be run by a player and not console (or your command is not specifying a player).

Try /sudo <player> before the command, if possible. Again, it's hard to deduce the issue without seeing the actual command.