With libraries like fullcalendar, I think importmaps often gets in the way more than it actually helps. The problem with fullcalendar, even once you get past this error, is that it internally references main.css which will then lead to the error

Failed to load module script: Expected a JavaScript module script but the server responded with a MIME type of "text/css". 
Strict MIME type checking is enforced for module scripts per HTML spec.

If your project requires something like FullCalendar or any JS library that includes and references CSS files, it may be better to use esbuild via jsbundling-rails

Regardless, if you want to experiment with this, you can pin fullcalendar with something like

bin/importmap pin @fullcalendar/core