What do you mean by not working? Processing fails? You do not obtain the result you expect? I am concerned that your formula could result in a negative number, which may make no sense for the calculated value.
Is the variable a standard FFCore variable (e.g., described in HELP)? Can provide me the name of the variable? Depending on variable type (e.g., string, enum, number, dimension), you may be forming a non-supported outcome with your formula. In what node are you applying the formula? Is the error posted on the job management GUI?
Unfortuneately your attached pngs are clipped. I do not understand how the formula is designated or the error message.
I see the problem. I was able to avoid the error by changing the equation order. For example - 6 + $FFvart$ (instead of $FFvart$ - 6 ) . Not a great solution but it worked for me.