In order to enable push notifications see related article for your device type.

iOS:

https://support.apple.com/en-au/HT201925

Android:

https://support.google.com/android/answer/9079661?hl=en

Windows:

https://support.office.com/en-us/article/set-push-notifications-65556daf-a9ba-493d-b6b8-236d823321e9