Apple watch 2nd generation not pairing with iPhone 6 iOS 12.5

I'm trying to pair an Apple Watch 2nd generation with the latest watchOS 6.3, with an iPhone 6 also with the latest iOS 12.5. When the pairing process starts, I get the message that the iPhone iOS is not up to date, requesting me to update the iPhone 6 iOS. However, when trying to update the iOS, it tells me it is up to date, it can not be updated.

Has anyone faced such problem? What is the workaround?

thanks