You can try to run softwareupdate -l in terminal to check for updates that way, but it should fail also and won't provide any more details. You need to look at /var/log/install.log and see why the update scan is failing, my bet is you have TLS filtering or firewall rules getting in the way of things.
You can try to run the Jamf Environment Test, or The Mac Evaluation Utility and see what they say is blocked.
https://support.apple.com/en-us/101555
https://learn.jamf.com/en-US/bundle/technical-articles/page/Network_Ports_Used_by_Jamf_Pro.html
Note: You can run software update -aiR to install OS updates from terminal, but you need to manually enter credentials for a volume ownership check. This prevents MDM's like Jamf from using CLI to install OS updates.
