The method toPushover() does not seem to exist on object<Illuminate\Notifications\Notification>.
This check looks for calls to methods that do not seem to exist on a given type.
It looks for the method on the type itself as well as in inherited classes or
implemented interfaces.
This is most likely a typographical error or the method has been renamed.
In PHP it is possible to write to properties without declaring them. For example, the following is perfectly valid PHP code:
Generally, it is a good practice to explictly declare properties to avoid accidental typos and provide IDE auto-completion: