Why can’t I upgrade my iPhone 7 Plus

The iPhone 7 series can only run up to iOS 15.7.2 currently.

Here's two different ways to ensure the highest iOS that your phone will support is on your phone.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> Update your iPhone or iPad - Apple Support

Update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support