Most issues with either a channel or PM import or both can be solved by the following articles.
Patch Management 8.x
First, ensure that you have installed the Patch point fix for your version (8.5 RU4, 8.6 RU2, and 8.6 RU3 have the point fixes available) to allow imports after changes by Red Hat: Red Hat Channel Import Failing
A common issue is that the user account specified for Red Hat in Settings > Software > Patch Management > Red Hat Settings > Red Hat Patch Remediation Settings policy > Red Hat Network tab is not in the Organizational Administrators group in the Red Hat portal: PMImport for Red Hat fails to download.
Additionally, see this article for similar issues with the Red Hat portal: Unable to download Red Hat Linux Channels to start Patch Management Import for Linux, or Patch Management Import fails.
Also in the Red Hat portal, the imports will fail if Simple Content Access is enabled: Red Hat Channel Import Failed - Rhsm: Entitlement isn't valid.
For issues where computers do not show applicable for expected errata, ensure Perl is installed on the endpoints: Red Hat Enterprise Linux Servers not populating under "Compliance by computer" and "Compliance by Erratum" for applicable Errata