ERR_NGROK_SHIP_APP_BUILD_CONFIG_NOT_ALLOWED: The 'build_config' field applies only to repository apps - ngrok documentation

What does ERR_NGROK_SHIP_APP_BUILD_CONFIG_NOT_ALLOWED mean?

ngrok returns ERR_NGROK_SHIP_APP_BUILD_CONFIG_NOT_ALLOWED with this message:

The 'build_config' field applies only to repository apps.

Where can I get more help with ERR_NGROK_SHIP_APP_BUILD_CONFIG_NOT_ALLOWED?

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