This message appears when (i) Solver has completed the maximum number of iterations, or trial solutions, allowed in the Iterations box 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 in the Iterations box, or click the Continue button instead of the Stop button in the Show Trial Solution dialog. But you should also consider whether re-scaling your model or adding constraints might reduce the total number of iterations required.

If you are solving a mixed-integer programming problem (any problem with integer constraints), this message is relatively unlikely to appear.  Solver uses the Maximum Subproblems and Maximum Feasible Solutions options to control the overall solution process; the count of iterations against which the Iteration limit is compared is reset on each new subproblem, so this limit usually is not reached.