LINGO supports cardinality sets of variables.  All variables belonging to a particular cardinality set must sum to a specified integer value.  To form a cardinbality set for variable X, Y and Z that sums to 2, you would specify the following in your model:

@CARD( 'MYSET', X);

@CARD( 'MYSET', Y);

@CARD( 'MYSET', Z);

@CARD( 'MYSET', 2);

You will receive this error message when the cardinality value, @CARD( 'MYSET', 2) in this case, is omitted.  You will need to add a cardinality value reference for the set.