I've got this error message. Could you help me out.
https://www.dropbox.com/s/onc77ig2cyv5ywo/Capture.PNG?dl=0
From the error message I can deduce that the game you were running uses AGSLua plugin, and the error is Lua error, not AGS or AGSScript error.
As far as I remember "attempt to index nil value" in Lua means that the variable is not assigned to anything while script tries to access a child element.
This is similar to "null pointer access" error in AGS Script (or any C-like language).
For example:
local parentVariable = nil
parentVariable.childVariable = 10
This code will cause "attempt to index nil value" error, because parentVariable was not assigned actual object.
PS. Also, I think I should mention that "Error message" is not a very good topic title. I recommend to change it to something that depicts the problem better (like "Error using Lua plugin").
This game gave me the error
http://www.escapistmagazine.com/content/indie-speed-run/?game=279
Oh... well, you really should tell such information from the very beginning... I thought you were speaking of a game you are making yourself.
Since this is a script error, no one will be able to fix this except game authors. I believe you should report this bug to them.
PS. I can note that an advise to report this on Advanced Tech forums is a wrong one... I wonder why AGS tells to do this when there is a lua plugin error.