We're aware of an issue affecting some players in Forza Horizon 4 when attempting to setup PvP, Co-Op, or Playground Games events. An error may appear saying "An Event Failed to Start." If you're in a convoy, the host may fail to join while the clients join successfully.
As a workaround for this issue: Once you receive the "Event Failed to Start" error, try joining the same event a second time and it should load successfully. Alternatively, the event should be visible in the activity panel and can also be joined from there.