LINGO allows you to define submodels within a main model using the SUBMODEL statement.  These submodels can be solved using the @SOLVE() function in Calc sections.  You have referenced a submodel that has not been defined.   Submodels must be defined in the model before any occurrences of references to them.  You may be able to solve this error by moving the submodel in question to an earlier position in the model text.