Hi, don't know what series Apple Watch but I bought it for my wife in August 2017 and now she has the iPhone 11 she is trying to sync them and it just says 16 hours to sync and it never finishes.
We have an Apple Watch series 0 that syncs with our iPhone 11 Pro. So it should work.
I think you aren't getting responses because you haven't given us enough information. What have you tried? Are both devices on charging when you are trying to sync? Did you try restarting both devices before the sync? Are bluetooth and wifi turned on on both?