Chrome Versions 73.0.3683.86 (released March 12, 2019) and above updated the flow for users to share their screen, and it's causing confusion for some users.
In order for a user to share their screen, they now need to take an extra step when when prompted to share their screen.
By default the option to share "Your Entire Screen" should be selected. If not the user needs to select it.
The user must now click on the screen preview itself.
This is an extra step that was not required before Chrome version 73.
The "Share" button will not be active until they click on the screen they want to share, even if they only have one screen.
After clicking on the screen, the background of the screen they selected will become grey (on a MacOS) or have a blue border (on Windows).
Now they can click on the Share button.
Here's what it looks like on MacOS:
and on Windows: