The latest PAN-OS software versions were
You can only see software versions that
are one feature release ahead of the current installed version.
For example, if you have an 8.1 release installed, only 9.0 releases
will be available to you. To see 9.1 releases, you first have to upgrade
(Panorama Virtual Appliance in Legacy Mode
only) The upgrade version failed to preload into the software manager.
This issue occurs when there are not enough
resources available. You can either increase the virtual machine
capacity or migrate from Legacy mode to Panorama mode.