Trying adding email account that you are using for Samsung account to google play store or google mail on the phone. Then remove the old email. Reset the watch and start pairing with the phone.
You might want to factory RESET the watch, and try the initial watch setup with the phone again. Watch the setup if it's transfering Samsung account during setup. It should be automatic. Also check your Galaxy wearable app on the phone if you login....tap on 3 horizontal lines to goto next screen and tap to picture at the top right corner to see your email info which will be link to the watch during setup. If you cannot setup, you might want to visit Samsung Store or Best Buy near you.