My apple watch (refurbished - in near perfect condition), Series 4, 40m refuses to connect to my iPhone 8, despite it being above the iOS requirements (12 < 16.7.8) and over the model requirement itself (5 < 8). I'm getting the error "iPhone Out of Date - This Apple Watch requires a newer version of iOS", a support agent in the Apple iMessage support even confirmed that my phone was above the requirements but couldn't tell me why it wasn't pairing correctly. (I have restarted both devices, I have factory reset the watch & setup restarted it).
#Apple Watch refuses to Pair
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The reason for this could be that your Apple Watch Series 4 could be on watchOS 10