iMac WiFi Hotspot not Working

You might have been sharing your connection from a wrong connection. For example, if you are connected via ethernet, and share it over wifi, you have to select the right ethernet port (some computers have more than one with USB or thunderbolt adapters).

Your network has some type of VPN set up, it prevents sharing. OR your ISP simply deployed hotspot request filtering technology. You either have to turn it off, hack it if you know how (looks like you don't), or just move on and not share your connection.

You hardware is broken. Nothing you can do there.

You encounter kext error. Need a full power down and restart.