This message appears if you select the Interval Global Solver engine and solve, and the Interpreter option is set to Excel Interpreter on the Task Pane Platform tab or if your model contains any non-smooth functions.  The Interval Global Solver considers the ‘special’ functions ABS, IF, MAX, MIN or SIGN non-smooth.  If you receive this message, create a Structure Report to pinpoint the exact cell formulas that are non-smooth; you must modify these formulas to use the Interval Global Solver.