Is your Apple Watch running on WatchOS 6.0? If that is the case it requires iOS 13 installed on the iPhone. You have an iPhone 6 and farthest it will go is iOs 12.4.4
In short, your Apple Watch (if it has WatchOS 6.0 installed) will not work with your iPhone 6. An Apple Watch with WatchOS 5 will work with your iPhone 6 and iOS 12.4.4
If the above is the case then you will need to downgrade your Apple Watch to WatchOS 5.0 OR Buy an iPhone that can run on iOS 13 (Which are iPhones 6s and newer models). There is no way that I know to downgrade an Apple Watch however you can try and get some help from the Apple Store.
Axel F.