Hello,
It sounds like you either don't have the Authorization header set, or it is set incorrectly. If you disabled the API entirely for your company, the error message would be along the lines of:
"The API has not been enabled. This is a setting available to your SAP Jam company administrator."
How are you setting your Authorization header?
If you are on the HANA Cloud Platform, I highly recommend using the HCP's built-in OAuth2SAMLBearerAssertion mechanism, which will manage the header for you based on the user currently logged into the HCP application. Setting this up with Jam is described in the following section of the guide:
Let me know how it goes!
-- Geoff