Check the iPhone network connection:
You need to have a good Service and/or Wi-Fi.
If they are On, turn Off Hotspot & VPN.
Charge the battery > 70%.
If you can't connect to the App Store, iTunes Store, or other Apple services
If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support
👉 If you tried all of the above steps and you still can’t connect, contact Apple Support.
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See the following Apple articles:
Set up your Apple Watch -> Set up your Apple Watch – Apple Support (UK)
The Apple Watch app -> The Apple Watch app – Apple Support (UK)
Apple Watch user guide -> https://books.apple.com/book/id6442854652