11. INVALID INPUT. A SYNTAX ERROR HAS OCCURRED.
This is the generic error issued by the LINGO compiler when it detects a syntax error. In Windows, when you close the error box, the cursor will be on the line where the error occurred. Other versions of LINGO will try print out the general area where the error has occurred, but LINGO cannot always pinpoint the exact line. Examine this area for any obvious syntax errors. If you are unable to find any obvious errors, a useful technique is to comment out small sections of the model until the error goes away. This should give you a good idea of exactly where the error is occurring.
Syntax errors may also occur if you are not invoking the correct compiler in LINGO. Most users will choose to build models using the native LINGO syntax, however, some users may prefer building their models using LINDO syntax. LINGO can compile models written in either native LINGO syntax or LINDO syntax. LINGO chooses the compiler based on a model's file extension. LINGO models must have an extension of lg4 (the default) or lng. LINDO models must have an ltx extension. The default model extension may be set by clicking on: LINGO | Options | Interface | File Format. Each model window's title bar displays whether it is a LINGO or LINDO model.