This message appears only if you press ESC or the Pause icon on the Output tab of the Solver Task Pane to display the Show Trial Solution dialog, and then click on the Stop button.  If you are controlling the Solver from a VBA program, remember that the user may press ESC while your VBA program is running.  You can write VBA code to disable the ESC key, or – better – define a VBA function that the Solver will call instead of displaying the Show Trial Solution dialog (an Evaluator in the object-oriented API, or a ShowRef function argument in the “traditional” SolverSolve function.).  Be sure to test for this OptimizeStatus or integer result (6) in your VBA code, and take action appropriate for your model.