PC Game pass not allowing mods

You could try following steps below to see if you're able to enable mods on a supported game:

https://answers.microsoft.com/en-us/xbox/forum/...

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today mate

Usually if a game supports mods, you are going to see a badge related to it
But I think the feature was temporary disabled, I was not able to see it too on my xbox pc app

It cannot be installed in any way. And it probably wouldn't work.

https://www.pcgamer.com/why-pc-games-should-never-become-universal-apps/

But in the future it might be possible. There is a planned change in the format of the windows app, to allow the use of WIN32

https://www.theverge.com/2019/5/30/18645250/microsoft-xbox-game-studios-publishing-valve-steam-32-bit-windows

Also in the last update of Xbox pass PC there are indications that mods will be allowed.

https://www.pcgamer.com/uk/mod-support-is-coming-to-games-on-the-microsoft-store/

I'm also waiting to use the Reshade in some games

The game only supports certain mods...a lot of mods won't work with it. Here's a couple of pages which may be useful:

https://www.nexusmods.com/news/14503

https://answers.microsoft.com/en-us/xbox/forum/all/modding-on-game-pass-pc-games/

As far as I'm aware modding of any sort on Xbox is a big no no. (Modding is an expected PC experience so tends to be allowed on some games)