8. The problem is too large for Solver to handle.

This message – or the more specific message Too many adjustable cells, Too many constraints, or Too many integer adjustable cells – appears when the Solver determines that your model is too large for the Solver engine that is selected (in the Solver engine dropdown list) at the time you click Solve.  You’ll have to select – or possibly install – another Solver engine appropriate for your problem, or else reduce the number of variables, constraints, or integer variables to proceed.

You can check the size (the number of variables, constraints, bounds, and integers) of the problem you have defined, and compare it to the size limits of the Solver engine you are using, by scrolling down to the bottom of the Task Pane Engine tab.  The problem size is also displayed in the Solver Model dialog.