If you are working with Studio One 2 Artist, you will need to install the latest version, 2.6.5. You will want to log into your my.presonus.com account and go to My Products>Software>Studio One 2 Artist to download the latest version. Double-click the installer to install. Once you are updated to the latest version, try activating once more. Try using the email address that is associated with your my.presonus.com account and not your user name. If you continue to have an issue, you will want to submit a support ticket from within your my.presonus.com account under "Support."
If you would like to try doing an offline activation, here is a link to a kb article that explains this process.