How do I delete all Smart Tags from the Catalog?

Upon further review, I see some possible answers why we are seeing different results with the Find>Untagged Items search.

I generally shoot both raw+jpeg images and automatically stack them upon import.  Usually, the raw file is the top image in the stack.  Here is what appears after I conduct the Untagged Items search:

Since both images are untagged, the stack is expanded to show them in the search results grid.  However, only the raw file has been tagged with Smart Tags. I have not yet determined whether a stack that has a jpg on top will be tagged with smart tags.  Presumably it will.

In any event, the raw file has an icon in the top left, showing that it is a Non Match for the search (because it has smart tags).

In the Options for the search bar, there are some drop-down choices:

If I choose Hide best match results, I get the same "error" message described in the OP.  (This toggles to "Show best match results").

If I choose Show results that do not match, my entire catalog is displayed.  This option makes no sense whatsoever except possibly as a toggle between all and best match choices.

To answer your original question, there is no easy way to delete smart tags.  It is possible to select your entire catalog and use the context menu to delete smart tags, one at a time, from the entire catalog.  That is an enormous and unwieldy task.  More reasonably, it is possible to select a batch of similar images and use the context menu to remove smart tags, one at a time, from the selected images.  Each smart tag you find and remove in a single image, will be removed from all of the selected images if contained within them.  But, again, this is a daunting task.

minnbrit  wrote Because once a smart tag has been applied the “Find/Untagged items” function does not find any photographs that have no personally applied tags.

I don't think that's true.  I just did a Find>Untagged Items search and the search results all have smart tags.  What OS are you using?

Instead I get an error message “No media available. Please clear applied filter and try again.”

You may have had the grid already filtered.  Please show us a screen shot of the entire window if you continue to get this message.  You should either be able to use the "clear" button on the top right of the grid, or the Back/Grid button on the top left above the grid.

I reviewed the documentation on smart tags and understand how to select and deselect in “Preferences/Media Analysis”.  However, in the documentation I did see a third option “Do not show Smart Tags and Auto Create”, but I couldn’t find that option in my preferences.

I think you may have been looking at the Elements 2018 documentation which has that option.  (There is no similar document that I am aware of for 2019.)