Hello John Yang,
Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with notifications on your iPhone. It looks like your iPhone needs updating to iOS 15.1. Do you have a recent backup for the data in your iPhone in iCloud or your computer? If not, we recommend that you create one as described here: How to back up your iPhone, iPad, and iPod touch
Then update your iOS as outlined here: Update your iPhone, iPad, or iPod touch
If the issue with notifications persists, check to make sure all of your apps are updated: How to manually update apps on your Apple device
Also, check out these two links for more help with resolving issues with notifications:
Use notifications on your iPhone, iPad, and iPod touch
Change notification settings on iPhone
We hope this helps.
Cheers!