https://www.makeuseof.com/tag/share-macs-internet-connection-using-os-x/…if you want to share your internet connection over Wi-Fi, your Mac needs to be connected to the internet via Ethernet, and not Wi-Fi.This is due to a limitation of the Mac’s internetnetwork device, which can either receive a Wi-Fi signal or send one out, but not both. Thus, you needto be receiving an internet connection through an Ethernet cable to use internet sharing.Mac使用的连网装置无法同时收发Wi-Fi讯号，因此若要经Wi-Fi共享网络，网络来源必须为有线(例如以太网路)