SAML Idp integration getting invalid access tokens


We’re currently trying to setup an IDP- initiated authentication using OKTA. The SAML integration appeared to be setup correctly but after logging in I get a POST request to my callback url with SAMLResponse. If I change the “Response Protocol” to from SAML to OpenID I then get an access token that appears invalid when parsing. Ideally I would like to get a valid access token sent to my callback url. What could I be missing?