A graphical front-end for command line emulators that hides the underlying operating system and is intended to be controlled with a joystick or gamepad.
libavformat56 is from Debian Jessie, Debian Stretch has 57 and 58, so you're using an outdated build of attract mode and it needs to be recompiled for Debian Stretch.
I am getting the below error while try to install attract mode on Raspbian Stretch:
attract: error while loading shared libraries: libavformat.so.56: cannot open shared object file: No such file or directory
libavformat.so.57 is installed on my system, but attract is expecting .56.