Widget Won’t Show Up

You really don’t give much information to enable helping.
You could say which widget, which theme, which WordPress version, where you tried to put it (from which page: editor or Widgets or Customizer), whether there was an error (did you check the console in Developer Tools or the error log on your server?), or if anything else wasn’t working.

My apologies for not providing adequate information. I am using the Oswald theme and the main sidebar widget. I have tried enabling the widget via the customizer and the dashboard. There has been no error that I am aware of, but am not exactly sure how to check that.

From your description it sounds like a widget custom to the theme? Or is that just a confusion? Widget Areas, for instance a sidebar, contain widgets such as Text Widget or Image Widget or RSS Widget.
If you have a sidebar widget, it must be your theme, and you should ask them.
If it’s core widgets in a normal theme sidebar, can you show a screenshot of the Widget admin page?

Here is the screenshot

OK, it looks to me like your theme is not outputting the sidebar correctly. There is a sidebar_left div, and it has an end div, but no widget. I was going to say that it could be getting an error in the sidebar output, but that would usually leave invalid HTML and it doesn’t have that.
I am certain that the core widget is working, so it must be your theme with the problem. It is best to ask at the theme’s support forum.

I will do that. Thank you for your help!