The possible causes are:

• The variable does not exist.• It is an axis variable, but the axis does not exist.
• It is an axis variable, but it does not exit for the axis type (linear or rotary).
• It is an axis variable, but it does not exist for the type of drive of the axis.
• The combined operator (+= -= *= /=) is not admitted for that variable.
• The value of the variable is not valid.

Check the program.