The Garmin Connect Challenges app does support every watch model, including the Forerunner 55 and 955, except for the Forerunner 255.
Given that there is no reason for this, I suppose Garmin just forgot to include the device in the supported devices list of the app.
The last update to the app was done almost three months ago, so Garmin, if you see this, please update the app to include the Forerunner 255.
