The warning "X11 has not been detected" is displayed on the Actiona window when the session it is running on does not use the X11 windowing system.

More and more Linux distributions are using another windowing system than X11. Ubuntu 17.10 for instance uses Wayland by default. Sadly, Wayland forbids any input emulation like mouse clicks or key presses for security reasons. It also prevents any program to interact with windows started by other programs. This means that when you start Actiona on a Wayland session many features will not work at all.

What can you do? For now Ubuntu provides a way to start a session using X11, see this link.

This is only a temporary solution because at one point X11 will be completely removed from Linux distributions. When this happens either the Wayland developers will have added an input emulation feature for programs, or Actiona will no longer work at all under Linux.