I forget this one over time, and it is annoying when you don't know or don't remember why its happening.
The message "Selected engine unavailable. Running script on XEngine" will be chatted to you when you specify in your script that it use an engine that isn't supported. XEngine is the default engine. The way that you specify something other than the default engine is by adding a comment on the first line of the script, in the format "//[specific engine name]:"
Unfortunately, that means that you will ALSO get that message anytime the very first line (line 0) in the script begins with "//" and ends with ":" (beginning with 2 forward slashes and ending with a colon).
So, if the very first line is "//This script does the following:" you get the message.
But, if you start your script with a blank line, actual script or any comment that doesn't end with ":" the next comment line can use that format. So "//This script does the following:" on the 2nd line won't generate that message.