Check this link for more detailed information:

Battery optimization settings can interfere with the way Viva gets location data from your phone and negatively affect the way Viva works.

In Android Marshmallow the battery optimization is part of the settings of the mobile phone. To disable battery optimization for the Viva app please follow these steps:

  1. Open Settings on your mobile phone
  2. Select the Battery section
  3. Select Battery optimization from the menu at the top right side of the screen
  4. Select Viva and choose Don't optimize