This is the message I see when I am trying to start matchmaking or a private incursion with a fried whom I gave the buddy pass.
He is playing on PC and I am playing on Stadia.
Both of us have cross-play enabled.
If he invites me and starts a matchmaking, we are able to play.
If he starts a private incursion, we get an error.
I can't even start any of them because they are grayed out.
What's happening?
Hey @krasorias Welcome to the Discussions!
Sorry to hear that you receive the error message when trying to play with your friend. If the options for you are not available to start a game, it seems that this may be related to your NAT. I would, therefore, advise you to make sure that there is nothing conflicting with your NAT type. We have a dedicated article for this which you can read here
Regarding the error message you mentioned, do both of you get the message, or is this only showing up on your side? Does it also display an error code, by any chance?
All the information would help us to look into this further for you! 
Hey there @krasorias,
Thank you for coming back to us with this information! If the issue persists after completing the steps previously provided by @Ubi-Dastan, please don't hesitate to let us know, so we can further look into this for you 
devtrackers.gg