You need to uninstall and reinstall the app, making sure to say 'Yes' when it asks permission to give push notifications.
If you're using an iOS device, go to Settings > Notifications > Find the app > Turn on 'Allow Notifications.'
If you're using an Android device, here are your options:
Option 1: In your Settings app
Tip: If you don't see "Recently sent," you're using an older Android version. Instead, tap App notifications and tap an app. You can turn notifications, notification dots, and notification categories on or off. If an app has categories, you can tap a category for more options.
Option 2: On a notification
Option 3: In the certain app
Many apps let you control notifications from a settings menu in the app. For example, an app could have a setting to choose the sound that app's notifications make. To make these changes, open the app and look for the settings menu.
Source: Help Center