Why can’t I set my phone with my watch series 3

The apple watch 3 GPS only has only 8GB of internal storage, most of that is taken up by the operating system and other critical software with not much space left for the update.So installing or even pairing to an ios14 or ios15 phone is proving nearly impossible for just about all the watch 3 users on this forum.

The same applies to Apple watch 1 and 2 they are almost impossible to either pair or update with an ios 14 or ios15 phone.

Your best bet is to call in at an apple store and ask them to update it for you.