Congratulations on your new iPhone! I understand that you did not unpair your Apple Watch before trading in your old iPhone, and now you're having some issues setting it up with your new device. I'm happy to provide some help for this issue.
Check out the relevant sections of the following article that I've highlighted for you:
Switch your Apple Watch to a new iPhone - Apple Support
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Your Apple Watch might still be paired with your old iPhone. Use the steps below to pair your devices.
You should erase your Apple Watch, then pair it with your new iPhone. If you’re asked to restore from a watch backup, select the most recent backup. You will lose Activity, Workouts, or settings on your watch that changed between the last backup and now.
Without an iPhone backup, you’ll need to set up your Apple Watch as new. You will lose any previous Activity, Workouts, or settings, but your other content will sync from the new iPhone to your Apple Watch.