Hi, Robert!
Yes, so since the plugin you want to use is an older one (and it is not compatible with the new editing experience in WordPress - which is sometimes referred to as “Gutenberg”) in order to use it you’d have to install the Classic Editor plugin. You don’t have to change your theme.
In the past, when building sites in WordPress you’d use the Classic Editor, but a few years ago there was the introduction of the block editor, which is what you’re seeing now. It is an editing experience that uses blocks to add page content and lets go of the traditional WordPress editing elements.
If you’re looking to learn more about the block editor, I absolutely recommend starting here:
In addition, Nick Diego hosts “Builder Basics” over on Learn WordPress & this particular Social Learning Space has lots of insight around basic design with WordPress:
Let me know if that helps or if you have any more questions!
Sam