The most common reason for an error when registering a new user is that the user's email address already exists in the Ploomes database.
If the email is already registered to another user (either in your account or in a trial account), an error message will appear in the bottom right corner of your screen.
To confirm whether the email address already exists in your database, go to Administration > User Table, and search for the email among both active and inactive users.
If you can't find the user, please contact Support so they can check the rest of the database.