Excel Solver - Stop chosen when the maximum number of [integer or feasible] solutions was reached (15).

If you are using the Evolutionary Solving method, this message appears when (i) Solver has found the number of feasible solutions allowed by the Maximum Feasible Solutions option in the Solver Options dialog and (ii) you clicked the Stop button when Solver displayed the Show Trial Solution dialog. You may increase the value of the Maximum Feasible Solutions option, or click the Continue button to continue the solution process.

If you are using the Simplex LP or GRG Nonlinear Solving method on a problem with integer constraints, this message appears when (i) Solver has found the number of integer feasible solutions allowed by the Maximum Feasible Solutions option and (ii) you clicked the Stop button when Solver displayed the Show Trial Solution dialog.  Again you may increase the value of the Maximum Feasible Solutions option, or click the Continue button to continue the solution process.  But you should also consider whether the problem is formulated correctly, and whether you can add constraints to “tighten” the formulation.