azure - SharePoint Error using Office 365 REST API OAuth access_token

I am developing a rather simple application that retrieves files from SharePoint using the REST Office 365 API. This is a batch job running in the background with no user interface so the following simple 2-step flow is what I'm doing:

(1) Requesting an Access Token from Azure AD (supplying client ID, client secret, resource, and grant_type=client_credentials)

(2) Invoke the SharePoint API (https://{base url}/_api/v1.0/Files) using the "Authorization: Bearer " as a request header .

That process seems pretty straightforward and sounds simple but I keep getting the following error:

401 Unauthorized

x-ms-diagnostics:  3001000;reason="There has been an error authenticating the request.";category="invalid_client"

Any idea what the problem is and how this can be resolved ? I have been stuck on this for days now. I would REALLY appreciate somebody's help with this. Thanks.