Can't connect to Forza Horizon 4 online

I have checked it, it is not my hardware and driver problem.

This problem will only occur if the xbox app is connected to the Internet.

This problem makes forza horizon 4 no way to play online mode.

If there is really no way to solve it, can I refund the Forza horizon 4 Ultimate Edition?

you can't join online play in forza horizon 4 until you have completed the first 4 seasons and got on the roster and reached level 20

Hello There ! I highly recommend following the troubleshooting steps mentioned on the link below:

https://support.xbox.com/en-US/help/games-apps/troubleshooting/troubleshoot-gameplay-issues-windows-10

We also recommend troubleshooting your network :

Troubleshoot your network connection speed | Xbox Support

You will need an active Gold subscription to be able to play online, aswell as a stable internet connection.

Forza Horizon 4 will automatically join the online lobby if you have a gold subscription.

Are you having any connection issues with other games as well, or is it specifically on Forza Horizon 4 only?

If you try a Horizon Solo session, and then try to switch online, does it give you the same error message?