Thank you for your post.
This issue is probably your General System Preferences. If I have "Show scroll bars" set to either "Automatically based on mouse or trackpad" or "When scrolling", the scroll bar will not appear until you start scrolling. If you want the scroll bar to always appear, and then set it to "Always".
In the first screen shot below, Show scroll bars is set to "Automatically based on mouse or trackpad". Notice there is no scroll bar. In the second screen shot, Show scroll bars is set to "Always". Notice the red arrow at top pointing to the scroll bar that now appears.
No scroll bar
Thanks for your prompt response.
No, I already had my "Show Scrollbars" set to "Always."
After posting this, I finally found the issue from an online post here:
It's caused by using the "Classic Theme" which was the default when it was created.
After changing the theme to one of the newer themes, which required restyling all elements, scrollbars appear and work properly.
I'll update my post above, accordingly, if this is possible.
No need to update your post above. If other customers run into a similar situation, then they will both your solution and my solution.