Apple Watch Series 4 is not connecting to my mom

This might happen because the iPhone might not be compatible with the Apple Watch and make sure your iPhone Bluetooth on but the Apple Watch or iPhone might be on a older version of iOS or watchOS so it is not compatible to see more check out this article! If your Apple Watch isn't connected or paired with your iPhone - Apple Support

If the Apple Watch does not have cellular capabilities family setup will not work. Also if the watch is running watchOS 10 (which is the newest compatible version) it needs an iPhone running iOS 17 or later. This happened to me before I got my new iPhone. Are you setting this up for your mom or for you?

What model iPhone is your mother using? What version of iOS is running on it?