We want to take code we developed in Salesforce and expose it as a REST web service an external application. The external system can use either Basic Authentication or OAuth 2.0 with
grant_type=client_credentials to authorize calls to any REST service.
I tried calling the REST endpoint in Salesforce via Basic authentication and I get
INVALID SESSION_ID error. I also tried oAuth with
grant_type=client_credentials and I get
unsupported_grant_type error. ( Note: I have configured OAuth 2 using connected app in Salesforce).
I have searched the Salesforce documentation. Am I missing some configuration in Salesforce?