ERR_NGROK_201: The ngrok API requires that you use Basic or Bearer authentication via the Authorization header - ngrok documentation

What does ERR_NGROK_201 mean?

ngrok returns ERR_NGROK_201 with this message:

The ngrok API requires that you use Basic or Bearer authentication via the Authorization header. It could not parse the header in this request. API keys and instructions are available on your dashboard: https://dashboard.ngrok.com/api-keys

Where can I get more help with ERR_NGROK_201?

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