could you send a screenshot of the exact error message?
The following method usually resolves the activation issue:
Please deactivate the plugin, and activate it again (not the license but the plugin itself in the plugins menu in WordPress admin).
Then please try to activate the license again.
If this does not solve the problem, please set the „Show License Activation Detailed Technical Information” checkbox in Settings / WP Related Items (WRI) menu.
After this, try activating the license again.
Right after the activation, you will find an „Activation technical information” section at the bottom of the activation page. Please copy the displayed information into an e-mail and send it to us.