This means that Solver has found solutions that satisfy the constraints, but it has been unable to further improve the objective, even though the tests for optimality (“Solver found a solution”) and convergence (“Solver converged to the current solution”) have not yet been satisfied. The exact meaning depends on whether you are solving a smooth nonlinear problem with the GRG Nonlinear Solving method, or a non-smooth problem with the Evolutionary Solving method.
When the GRG Nonlinear Solving method is being used, this message occurs very rarely. It means that the model is degenerate and Solver is probably cycling. One possibility worth checking is that some of your constraints are redundant, and should be removed. For more information, see the discussion of GRG Nonlinear Solving Method Stopping Conditions.
When the Evolutionary Solving method is being used, this message is much more common. It means that Solver has been unable to find a new, better member of the population whose “fitness” represents a significant relative improvement over the current best member’s fitness, in the amount of time specified by the Maximum Time without Improvement option in the Solver Options dialog. Since the Evolutionary Solving method has no way of testing for optimality, it will normally stop with either “Solver converged to the current solution” or “Solver cannot improve the current solution” if you let it run for long enough. If you believe that this message is appearing prematurely, you can increase the amount of time allowed by the Maximum Time without Improvement option. For more information, see the discussion of Evolutionary Solving Method Stopping Conditions.