Solver Error: "This workbook is currently referenced by another workbook and cannot be closed"

To Reset Solver Add-in in Excel

1. From the File Menu, select Options
   
   
   
   
2. Select Add-ins in the left navigation then at the bottom of the windows, ensure the "Manage:" drop-down displays Excel Add-ins and click on Go.
   
   
   
   
3. Uncheck all the boxes, then click OK
   
   
   
   
4. Exit Excel
5. Open Excel and a blank workbook
6. Repeat steps 1-4 rechecking the boxes that were previously unchecked