Variable Problem!!!!!!!

[Gold Soul Demon Ranger]

I need to use variables in my game, but whenever I test play and try to activate the event with the variable it comes up with this,

Script 'Game_Interpreter' line 750: NoMethodError Occurred.
Undefined Method '[]' for nil:NilClass

If anyone can help me that would be great.

[Arajuna]

What is the event for like what does it do?