First-time GreenSock user here struggling to fix this error message too. I'm trying (and prefer) NPM install and ES module. I have
import { gsap } from "gsap";
// ...
let el_first = el_div_with_photos.firstChild;
gsap.to(el_first, { x: 200, duration: 2 });
... which gives this error in Chrome's console: "gsap-core.js:87 Invalid property x set to 200 Missing plugin? gsap.registerPlugin()".
I am running gsap 3.7.1 installed via NPM. Hugo (with its built-in ESbuild) bundles my JS. AlpineJS and AnchorJS are the only other libraries active on the page. The NPM setup must be okay otherwise gsap wouldn't get invoked. As you said, "no imports necessary". But still, I tried importing CSSPlugin and gsap.registerPlugin(CSSPlugin) just in case the NPM/import-way of no longer auto-registered CSSPlugin (as that plugin might not be needed or desired in some use cases). Still get the same error. Out of ideas here :(.