ERR_NGROK_4114: The email domain "DOMAIN" does not allow self-serve user registrations - ngrok documentation

What does ERR_NGROK_4114 mean?

ngrok returns ERR_NGROK_4114 with this message:

The email domain "<DOMAIN>" does not allow self-serve user registrations. Contact your ngrok administrator or support@ngrok.com for access.

Where can I get more help with ERR_NGROK_4114?

Email support@ngrok.com and include the error code.