One of these messages appears when you click on a button if the Set Cell edit box contains something which cannot serve as the objective function in a Solver problem. The Set Cell must refer to a single calculated value, on the active sheet, which is to be maximized, minimized or made equal to the target value. In order for the Solver to change the value of the Set Cell, it must contain a formula which depends on cells on the active sheet, some of which must be adjustable cells (in the By Changing Cells edit box). If you receive one of these messages, check and modify your Set Cell selection and try again.