If you are receiving a message that reads “An error occurred during account creation. Please contact the system administrator for more information.”, it means the email address being used is in your block list. To remove this go to General Settings > Fraud Prevention > Signup Blocking (tab) in your admin center and remove the email address from the ban list.
If you didn’t manually add this email to this ban list and you’re wondering how it got there, it’s because you have the feature Ban Email When Account Is Terminated on this same page selected and an account with this same email address has previously been terminated/removed/deleted. When it was terminated, the email address was automatically added.