“I cannot communicate with Engine” means something has gone wrong with the system inter-process communication system that Keyboard Maestro and Keyboard Maestro Engine use. This will break a lot of things in Keyboard Maestro, for example any changes to your macros will not be noticed the the engine, and same for changes to variables or clipboards, recording wont work, etc etc. All sorts of things.
Either restarting the Keyboard Maestro Engine or the system is generally required to resolve this.
The issue is a system failing so there is not much I can do about it, and it happens often enough that the Accessibility system detects and reports this. It may be related to the low level locking system crash that is present in 10.12 and 10.13, but thankfully fixed in 10.14, which crash is responsible for roughly 95+% of all Keyboard Maestro Engine crashes (it is still very rare, roughly only once per 600 user-days, but Keyboard Maestro would crash roughly once per 10,000 user-days without that system bug).