Unable to pair Apple Watch 5 with iPhone 7 Plus iOS 15.8

In order for your watch to pair with your iPhone, it will need to be running watchOS 6, 7, or 8. If it’s running watchOS 9 or 10, you’ll need a newer phone to pair it with.

What do you mean by “Keep rolling back”?

From If you can't pair an Apple Watch with your iPhone - Apple Support:

If it seems like the pairing process has stalled — for example, the pairing animation remains on the screen for several minutes — resetting your Apple Watch might help:

1. Press and hold the Digital Crown while your Apple Watch is in pairing mode.
2. Tap Reset when it appears on your watch.
3. After your watch resets, you can pair again.