Failure creating a token for auth0-authorization-extension-api

Trying to get a token for authentication API access, I get

{"code":404,"message":"unable to resolve jtn to webtask token","req_id":"1550684083612.753095"}paul:dev$ 

The request is done through curl:

+ curl --request POST --url https://idigitalasset.us.webtask.io/adf6e2f2b84784b57522e3b19dfc9201/oauth/token \
--header 'content-type: application/json' \
--data '{"grant_type":"client_credentials","client_id": "<hidden>","client_secret": "<hidden>","audience": "urn:auth0-authz-api"}' 

I can find no solution to this in documentation - help is appreciated. The request looks right…