Not all applications in the Enterprise App Gallery have provisioning enabled. This either means that the app does not support SCIM, or that the app may support SCIM but we have not yet received or finished processing a request to add provisioning directly to the gallery app.
You should be able to use the custom non-gallery app option when adding an application to add a generic SAML/SCIM app that you can configure SCIM on, targeting it to any functional and SCIM-compliant endpoint.