If you click on that error, there should be a popup with a more detailed
message - what does it say?
Also see here:
http://www.tiddlywiki.org/wiki/Troubleshooting
-- F.
Sorry for the delayed response - I had to figure out where to get the
plugin to do some testing (that's why the troubleshooting page
recommends you supply links to the relevant plugins... ).
I've imported the ToggleTagPlugin* into a fresh TiddlyWiki (v2.4.0), and
it seems to work just fine. Maybe you have an old version?
If not, please upload a simple demo somewhere (e.g. on Tiddlyspot)?
-- F.