Due to recent Steam VR v1.9.15 change in the VR launcher, VR user will encounter cannot use exit game/call service when press system button on the controller.
The long-term solution for fixing this issue is released in VIVEPORT Arcade Manager Client version 126.96.36.199 [updated 2020/01/09], please check your client version and update accordingly via SETTINGS>>VERSION on VAM host.
If you do not want to update your client version, you can refer to below workaround before the fix:
When the launcher is up, click the System button on the controller
Click VIVEPORT button (highlighted red)
Enter Game, click System button again, and you will be able to launch Arcade Dashboard now