Hi, I got an used series 6 3 weeks ago and unfortunately discovered I couldn't pair it with my iphone (SE3) unless I updated the phone to ios 17. I had the same error message you got even though the watch had watchos 9 and the phone had ios 16, both of which are compatible. I was forced to update to make it pair.
Possibly, try to contact apple support to check if there's any workaround?