Some issues with the Marlin App may be related to the permission that is required for the App to access certain resources. This applies to all Android version after 6.0 (Marshmallow).
The Marlin needs access to Location for Bluetooth Access and Storage for data synchronization. The settings can be accessed by selecting the following menu under Setting -> Applications -> Application manager -> Marlin -> Permissions
Once the permission has been enabled, please restart the App and try again.