So sorry you are experiencing the issue, my PHP version is 5.6, the module works fine on my demo site. please send me your backoffice access and FTP in a private reply, I will investigate the issue for you.
The webp images were generated in your store, see attached screenshot. but your theme is older, please upgrade the theme to the latest version to apply the webp feature.
IMPORTANT don’t forget making full files and database backup before any update.