User account licensing (logging in when activating) and by extension the invitation function doesn't work if you are using a very old version of a product (prior to 2017) or an unsupported tool (eg. MySQL Compare) so if you have been invited to use a license in the customer portal and you try to activate it you will get the error message:
To activate the license you will need to log in to the customer portal and remove yourself/the user. Then you can install the license using a product installer which you can download from the website and activating the license key.
To remove an allocated user, click on the Remove button alongside their details in the list.