This message means that the internal “model” (information about the variable cells, objective, constraints, Solver options, etc.) is not in a valid form. An “empty” or incomplete Solver model, or a corrupted Excel workbook can cause this message to appear. You might also receive this message if you are using the wrong version of the Solver files, or if you’ve modified the values of certain hidden defined names used by Solver. To guard against this last possibility, you should avoid using any defined names beginning with “solver” in your own application.