The method assemble() does not exist on Namshi\Notificator\NotificationInterface. It seems like you code against a sub-type of Namshi\Notificator\NotificationInterface such as Gewaer\Contracts\PushNotifications or Gewaer\Notifications\Mobile\Mobile.
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation