18. All variables must have both upper and lower bounds.

If you are using the Interval Global Solver or the OptQuest Solver, this message appears if you have not defined lower and upper bounds on all of the decision variables in the problem.  If you are using the Evolutionary Solver or the multistart methods for global optimization, and you have checked the box “Require Bounds on Variables” on the Engine tab of the Solver Task Pane (it is set to True by default), this message will also appear.  You should add the missing bounds and try again.  Lower bounds of zero can be applied to all variables by setting the Assume Non-Negative option to True on the Engine tab in the Solver Task Pane or by entering “0” for the Decision Vars Lower option on the Platform tab of the Solver Task Pane.  Non-zero upper or lower bounds can be added as constraints in the Model tab of the Solver Task Pane or by entering a value for Decision Vars Upper or Decision Vars Lower on the Platform tab in the Solver Task Pane.  You must define bounds on all variables in order to use the Interval Global Solver or the OptQuest Solver.  For the Evolutionary Solver or the multistart methods, such bounds are not absolutely required (you can uncheck the box “Require Bounds on Variables”), but they are a practical necessity if you want the Solver to find good solutions in a reasonable amount of time.