CVAD Administrator cannot select application source: MSIX; it is greyed out or not available.
Example from 2402 version:
Example from 2507 version:
Additional information:
MSIX and MSIX attach apps require a minimum functional level of 2106 or later, while FlexApp apps require 2206 or later.
Check functional level and change it if required.
In WebStudio:

If change is required:

Make sure that all machines in Delivery Group have VDA version installed which is equal or newer than functional level.
You can find more information here: https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/2507-ltsr/install-configure/delivery-groups-manage.html#change-the-functional-level
Workaround:

CVAD Administrator cannot select application source: MSIX when he tries to publish application to the Delivery Group.
There are MSIX packages available when you check App Packages node in Web Studio.
The option is available when you select different delivery group.
Both Delivery Groups have machines with the same VDA software version installed.