It looks like you already tried to register an account, as this message will only appear when the keypair you’re using has already been registered. This can happen if you don’t use a proper folder structure (this is important if you have multiple sites on your server).
In the default configuration, you need to always have the root directory (where your PHP files reside in) nested in another directory specific to that site, for example /sites/my-site-1/html/ and /sites/my-site-2/html. If you have your directories set up like /sites/my-site-1-html/ and /sites/my-site-2-html/, you will run into issues since the certificate directory always resides one level above the site’s root directory (which in the second case would be the same location).
If you have indeed a setup like the latter, you need to override the certificate directory location, by defining the constant WP_ENCRYPT_SSL_CERTIFICATES_DIR_PATH for each site and filling in the location where the certificate should be stored.
