How do I connect my iwatch series 3 (8.7.1) with iPhone SE2 (16.0.3)

i recently bought watch series 3, I am not able to update or pair with my iPhone SE 2 which runs in 16.0.3 iOS

when I try to update it shows estimation time of 2-3 hours once it completes either it says preparing or it shows a pop up saying unable to verify update