An educational CostX licence can only ever be activated once, on one computer. As you’ve tried to uninstall and reinstall again, you’ll need to get a new serial number.
Firstly, completely uninstall CostX from your current computer, including selecting ‘No’ when asked if you wish to keep the database.
Then, please apply for a new licence at the following link: http://techweb.exactal.com/resources/educational_request.php
Once installation is complete, you should be able to use your new licence with no issues.
Hope this helps!
CostX Forum Admin