ERR_NGROK_16000: The email of the user you are trying to add, EMAIL, does not use a domain verified for your account - ngrok documentation

What does ERR_NGROK_16000 mean?

ngrok returns ERR_NGROK_16000 with this message:

The email of the user you are trying to add, <EMAIL>, does not use a domain verified for your account.

Where can I get more help with ERR_NGROK_16000?

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