Help Needed - unsolved "MSIXVC packages are only supported for products that use the modular submission model"

Question

I tried to follow your documentation on how to make a package to upload to Partner Center but with no success.
After spending days searching why and how this does'nt work I finally decided to give up and ask for help as informations seem to not exist.
So, following the steps on https://learn.microsoft.com/en-us/gaming/gdk/_content/gc/get-started-with-pc-dev/get-started-with-unity-pc/features/gdk-unity-build-and-run
I finally get a build with no error in validator (which should be out of the box but is not)
and then when I try to upload it on Partner Center it leads me with this error:
"MSIXVC packages are only supported for products that use the modular submission model"
Of course there is no information anywhere on the "modular submission model", this is why I am asking it here.
What should I do to be able to submit a package?
And also is it the only way to configurer the store presence of our game? this seems to be very user unfriendly compared to every other platforms (e.g. Steam, Epic or any non microsoft console)