my simple notification extension is outdated, see also this note on my webpage 404 Not Found | Pura Vida Apps
This is a simple notification extension and currently only runs on older Android versions, i.e. < Android 8. Therefore my recommendation is to use one of the other available notification extensions, see also here.
my suggestion for you is to try the one signal extension
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.