Formula select variable

[nek11man]

Hi there, hope someone can give me a hand, i m a newbie to this and no background in coding at all, and i have a simple question.

I have a simple formula, forceindex of candle0 MINUS forceindex ofcandle1, after i click update, there s a popup "you must select a variable", what exactly is the next step i need to do?

thank you

[fxDreema]

http://prntscr.com/8g91sf