I can’t update my Apple Watch

See -> Update your Apple Watch -> Update your Apple Watch – Apple Support (UK)

• Leave your Apple Watch on its charger while it's updating. Don't restart your iPhone or Apple Watch, and don't close the Apple Watch app. When the update has completed, your Apple Watch will restart on its own.

Your watch is running watchOS 8 - and it is very likely that your iPhone is running iOS 15.x. If you cannot update your iPhone beyond iOS 15.7.x, then you have either iPhone 6S, 7 or SE (1st generation). watchOS 9 requires iOS 16 or later.

These iPhones cannot update to iOS 16 due to your internal hardware being too old to support it.

Apple ended major software update support for your iPhone in September 2022 - although it continues to receive security updates - with the most recent being iOS 15.7.6, released in May 2023

These iPhones are approximately 7 years old.

iPhone 8 and later models are capable of supporting iOS 16.

You may consider buying a new iPhone. Even the entry-level iPhone SE can run the latest iOS version, iOS 16.5.

Here is a list of unsupported iPhones with max iOS supported:

Model            Architecture  Introduced       Max supported iOS
iPhone (1st)     32-bit        June 2007        iOS 3.1.3
iPhone 3G        32-bit        July 2008        iOS 4.2.1
iPhone 3GS       32-bit        June 2009        iOS 6.1.6
iPhone 4         32-bit        June 2010        iOS 7.1.2
iPhone 4S        32-bit        October 2011     iOS 9.3.6
iPhone 5         32-bit        September 2012   iOS 10.3.4
iPhone 5C        32-bit        September 2013   iOS 10.3.3
iPhone 5S        64-bit        September 2013   iOS 12.5.7
iPhone 6         64-bit        September 2014   iOS 12.5.7
iPhone 6 Plus    64-bit        September 2014   iOS 12.5.7
iPhone 6S        64-bit        September 2015   iOS 15.7.6 <- Likely your iPhone model
iPhone 6S Plus   64-bit        September 2015   iOS 15.7.6 <- Likely your iPhone model
iPhone SE (1st)  64-bit        March 2016       iOS 15.7.6 <- Likely your iPhone model
iPhone 7         64-bit        September 2016   iOS 15.7.6 <- Likely your iPhone model
iPhone 7 Plus    64-bit        September 2016   iOS 15.7.6 <- Likely your iPhone model