Cannot share WiFi on my MacBook

If you are connected to the Internet using WiFi you can not share the connection using WiFi. Connect to the Internet using Ethernet then share the connection using WiFi. WiFi can only connect to one network at a time.

macOS Sierra: Share the Internet connection on your Mac

When you enable Internet Sharing, you are basically turning your notebook into a software router. As such, you can only share your Mac's wireless connection with any of the remaining available ports on your device. From your screen shot, your only choices for those ports would be either a Thunderbolt Bridge or via a Bluetooth PAN. On the other hand if you connected your notebook, via one of those connection, for Internet service, you could then share out it Wi-Fi port for other wireless clients.

OS X Yosemite: Share your Internet connection - Apple Support