Not able to share internet from MacBook Pro

Are your cell phones or other devices connected to the same LAN as the Mac? If so it would be a very unusual network configuration.

Refer to Share the internet connection on Mac with other network users - Apple Support, the key words being other network users. In other words a Mac connected to the same LAN but over a different interface. For example you could share an Ethernet Internet connection over Wi-Fi or vice versa.

Can you post a picture of that Sharing pane?

Um, do your HAVE a working connection coming in on your 10/100 LAN?

is 10/100 LAN TOPMOST on the list of network interfaces?

I don't have an answer for you yet. I got as far as sharing an Ethernet connection over Wi-Fi:

That worked. However, when I chose to share the iPhone USB connection over Wi-Fi (not shown in the above screenshot) that's when things went sideways. I sent my employees home for the rest of the week. They're happy but it was not an experience I want to repeat.

Maybe when I recover from the trauma I'll have something useful to offer. In the meantime Grant Bennet-Alder asked some relevant questions. No I don't believe resetting your Mac is justified; most likely something else is going on that resetting the Mac won't help.

EdIphone20 I haven't had an opportunity to perform any more testing than I did previously, which turned out to be unexpectedly disastrous... probably due to the complexity of the networks I typically use. What I need to do is to create a simple network configuration to determine what you need to do to accomplish what you seek, and I just haven't found the time.

I don't like it when people don't follow up when they say they will, and I wanted to let you know even though I have nothing to add quite yet. Don't reset your Mac. I doubt anything is wrong that resetting the Mac could possibly fix.

Thanks. In attempting to duplicate your conditions I crashed my network 😆 so I'm posting from my iPhone. I'll have more information when I get things running again.