What error message do you receive when you attempt this activation? Please quote the wording in full or, better still, show us a screenshot of the message.
If the problem is that you have already activated your software twice, there is a limit of two activations being possible at the same time. In order to resolve this you can go to your Magix account, log in, go to My products and find the listing for whichever version of Web Designer you have. Click on Show more details and deactivate any installation you no longer want or need. Log out, wait a few minutes and then try again to activate your new installation.