Error with action hub: invalid Authorization header

[Greta991]

Hi,
i followed this guidelines: actions.
I created my custom Action Hub on Heroku and now i'm trying to attach it to the Looker instance but i get this error:

I have already set the following variables:
ACTION_HUB_BASE_URL = "https://my-heroku-action-server-1234.herokuapp.com"
ACTION_HUB_LABEL = "My Action Hub"
ACTION_HUB_SECRET = "XXXXXX"

Why do i keep getting this authorization error?
Thank you