I just got mine to work! It was a Series 1 watch with an iphone 7 plus on ios 14.
I can't tell exactly what did it (I tried a lot of stuff) but it was some combination of the following:
factory reset the watch
download the newest software update on the iphone
reset the phone
manually pair the watch to the phone
I also had to do the "reset everything" stuff twice - once to get the watch's update to download onto the phone, and a second time to get it to update the watch. After all of that though, it worked!
I called Apple support to help and they stuck with me through it all, they were really great :) Also, both update downloads took a really long time so I just had to be patient