Foreman nightly, while deploying a new host, creation fail with the error on topic.
If I try to render Kickstart default iPXE template on Host -> Provisioning Templates, I get the same error as above.
The medium_uri method for operating systems has been deprecated in 1.20 and removed in 1.24. Please use medium_uri macro instead.
Thank you for testing with nightlies and reporting this issue! Looks like the some of the community templates have not yet been updated to match this change. I opened Bug #28150: Some community still use removed @host.operatingsystem.medium_uri method - Foreman to make sure we fix this in time for the 1.24.0 release.