Thanks for your post.
An educational CostX licence can only ever be activated once, on one computer. As you’ve tried to uninstall and reinstall again on the same computer, 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: https://education.itwocostx.com/support/forum/educational-request/ (or click the red button ‘Apply for a Licence’ at the top of this page)
Once the licence is approved, you will be emailed installation instructions which you should carefully follow.
Once installation is complete, you should be able to use your new licence with no issues.
Hope this helps!
CostX Forum Admin