cache not working / not detected ?

Hi,

I am not sure the plugin is working : WordPress doesn’t detect the plugin (Health Site tells me that your plugin is not detected and pages are not cached ), and I don’t see any “WP fastest cache was created in…. seconds” in my source code.
url : https://feebf.com/
(you only can view a coming soon page for the moment).
I cleared every cache I could think of.
Thanks for your help.