Excel Solver - Stop chosen when the maximum number of [integer] subproblems was reached (16).

If you are using the Evolutionary Solving method, this message appears when (i) Solver has explored the number of subproblems allowed by the Maximum Subproblems 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 Subproblems 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 explored the maximum number of subproblems (each one is a “regular” Solver problem with additional bounds on the variables) allowed by the Maximum Subproblems 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 Subproblems 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.