Hi @mrraffus, thanks for getting in touch!
Problems might arise if you attempt to install your license after 24 hours from generation, you will no longer be able to automatically install the Wordfence license key. Additionally, if you are in a different browser than the one used when requesting your Free Wordfence license, you will be unable to automatically install your new Free Wordfence license. In those cases you will need to manually copy and paste the key from the email to complete the activation of Wordfence Security.
Aside from verifying that you did not inadvertently copy only part of the license key, I would check whether you can install the license when Wordfence is the only active plugin on your site. There could be a Javascript conflict with another plugin potentially stopping the verification of the key from communicating with our servers.
Let me know if it still doesn’t work after trying those things.
Peter.
