This message appears when (i) the Solver has run for the maximum time (number of seconds) allowed in the Max Time field on the Engine tab of the Solver Task Pane and (ii) you clicked on the Stop button when the Solver displayed the Show Trial Solution dialog. You may increase the value in the Max Time box or click on 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 solution time required.