Thumbnails not displaying

I’m setting this up on a website to take the place of another plugin that’s no longer supported. I like the customization options, however, no thumbnail is displayed even if there is one present or if I select a default thumbnail.

In troubleshooting I have tried switching to a default WordPress theme and the thumbnails still do not display. Thumbnails do display in the widget that I am replacing, so I know they are configured properly. Can you assist with additional troubleshooting?

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