Android: TeamViewer -> Settings -> Version.
When you see a pop-up offering you to Update Now, please check the version of the TeamViewer software you are currently using, and immediately update to the latest version of TeamViewer. TeamViewer recommends that all users download and start using the latest version of TeamViewer on all supported operating system platforms. This article applies to all TeamViewer users who have not upgraded the TeamViewer software to the latest version and see the error message: 'Unable to connect - Reason unknown'.