2 things must be confirmed:
1) login in USS with an admin account and
a) Go to configuration >> Data sources >> CA Service Desk and For the "Service Desk REST URL FOR MOBILE" test connection.
If the connection is successful then check if the server name is responding correctly by doing a ping from CMD command prompt window.
In this case, the server name must be replaced with FQDN and try test connection again.
Example: http://TESTSERVER:8050 for http://TESTSERVER.yourdomain.com:8050
Then try connection from mobile and see if message alert still appears.
2) if message alert is still showing in mobile screen then confirm that this URL +/workflow can be accessed from the browser.
If page response with "Help for Workflow Manager Mobile App" then the application is working but there might be an issue with DNS so the workaround for this case is to replace your server name with the IP in USS>> Configuration>> Data sources >> CA Service Desk >>"SERVICE DESK REST URL FOR MOBILE"
test connection and if is successful then save changes, delete data and cache from CA Service Manage app in mobile and try connecting again.
This time message shouldn't appear again and all options must be available.