That error message in from the plugin code itself. When invoked it checks to make sure that all the expected files and paths have been properly provided. These hadn't been changed in a very long time until quite recently. And that change happed AFTER Sigil-0.9.18 was released.
Would you please try downloading a fresh flightcrew plugin from here (version 0.9.2):
https://github.com/Sigil-Ebook/flightcrew/releases
Then in Sigil use ManagePlugins to remove flightcrew, then add in the fresh copy of the flightcrew plugin zip archive that you just downloaded.
Please give that a try. If you get the same error, then we will need to dig deeper.
KevinH