To receive post notifications from the App, you must enable them in your device settings. Please go to your phone's "Settings" screen and check if notifications for this App are turned on.
For iOS settings, please refer here.(https://support.apple.com/ja-jp/HT201925)
For Android settings, please refer here.(https://support.google.com/android/answer/9079661?hl=ja%20-%20zippy=)