Hangouts Notification - SMS no longer supported
I think you are getting this message again and again because Hangout is still registered as the default app to handle SMS messages (which it doesn't support anymore). 

Therefore have to make a different app the default SMS app. To do so open the Android Settings app (the gear icon) and select the Apps and Notifications section (might be named a little different on your device depending on the Android version and manufacturer).

There you will find an entry that allows you to select the Default SMS app. If you can't choose the app your device may be missing an app that can handle SMS. In such a case search the Google PlayStore for an new SMS app.