Comment 4 for bug 2056627

Its not just that app images don't have a default path, we can handle that as well. It is that user namespaces have become a privileged operation, and the user must take some privileged action to allow applications to use them.

That can be any of
- moving the application into a well known privileged location that has a profile already associated with it.
- creating a profile for the application where it is installed in their unprivileged location. This is currently allowed but problematic in that unprivileged code code potentially write to it and we are not currently restricting unprivileged applications from writing these locations. But that will come
- tagging the application with the correct security label.

The important part is the user must take a privileged action to allow applications that are using user namespaces to gain privilege. Note, applications that use user namespaces that don't require privilege are allowed, its only applications that require privilege within the user namespace.

Unfortunately appimages that use use namespaces need the user to take one of the above privileged actions. And unfortunately Ubuntu can not "fix" this without disabling the protection. There are plans to improve the user experience and make this easier for users to do, but atm it is a manual process.

The instructions provided by Seth will enable you to get the appimage running.