I went to upgrade my Xanadu instance to v27.1.2 and was surprised that I could not find v27 in the App Manager.  The issue is the patch level of your instance.  This new version requires Xanadu Patch 7+ or Vancouver Patch 1+ and App Manager will not show you versions which are incompatible with your instance. 

Go to https://support.servicenow.com to upgrade your instance then give it a few minutes for the data to propagate in the backend before using the Sync button from the App Manager home screen to get the updated list of versions.