I’m not sure of your exact problem yet, but I would start with setting up your service configuration in Windows Dev Center, if your game is a UWP game you won’t use XDP at all. Again, I was confused on this too. Also, when you set up your game in Windows Dev Center you have to create it as a sandbox project if I remember right (I just know I had to delete and recreate mine).
I would reach out to IDDS, they are the ones in charge of making sure you’re set up properly in Dev Center. Once that’s done you can see where you stand.
Also, in your code when AddUser(XboxLiveUser user) gets called is that user already logged in? It’s apparently VERY IMPORTANT (for what reason I don’t know) that when you create the XboxLiveContext the user is already signed in. If not, parts of Xbox Live will work and other parts won’t, I struggled with this for days before figuring it out.
https://forums.xboxlive.com/questions/65290/rich-presence-setpresenceasync-throws-bad-request.html