Why does my Apple Watch keep unpairing when I try and set it up with my phone? It is an iPhone 6

So....couple of questions: (1) what iOS are you running on your iPhone 6; (2) what model Apple Watch do you have (Series __(1, 2, 3, 4 or 5) and (GPS only or GPS + cell); and (3) what Watch OS are you running?

Those factors can impact compatibility. There's some good information regarding this in a prior thread.

If you are sure they should be compatible, then you need to start trying the pairing process from scratch and provide more detail as to where in the process breaks down and what, if any, error messages you receive.

-- JDee

Sounds like they are not compatible. :-(. According to the Apple Series 5 specifications page (at the very bottom), Apple Watch Series 5 require an iPhone 6s or later with iOS 13 or later.

-- JDee

Which model of watch is it?.

Which version of watchOS it the watch running?

An apple watch 5 requires a phone with ios13 which is an iphone 6s or higher your iphone 6 will only update to ios12 so the watch won't pair with your phone.