Apple Watch Series 6 will not pair with the phone

The checks in more detail

If the watch shows a message about needing the previous owner's details, only they can remove it. A watch that is stuck in an old state may need a force restart and an erase. If the iPhone camera cannot read the pairing animation on the watch, pair manually. Make sure the iPhone supports the watchOS the watch ships with, since a new iPhone with a very recent iOS pairs with a Series 6 on watchOS 11 without trouble. Restart both devices and keep them close. A very low battery can stall setup, so charge the watch first.

If you bought the Series 6 second-hand from a trusted seller, ask them to unpair it from their iPhone and to remove it from Find My before they hand it over; this is the surest way to avoid Activation Lock. A seller can check this in the Find My app under Devices. A watch that is still listed there cannot be set up by you. If pairing keeps failing at the same step, note exactly where it stops, such as at the Wi-Fi step or the update step, because each points to a different cause:

a Wi-Fi step points at the network, the update step points at storage or a slow download, and the activation step points at Activation Lock or the carrier. Writing the step down speeds up a support call and avoids repeating checks that do not apply. If the pairing completes but the watch then shows a message that it needs to be updated, install the update on charge and wait for it to finish; the watch may restart several times. A Series 6 with an out-of-date watchOS from storage can take a while to update, particularly over slow Wi-Fi, and an interrupted update can leave it unable to pair, which a force restart usually clears.