Minified React error #321; visit https://reactjs.org/docs/error-decoder.html?invariant=321
One reason could be you are importing useEffect from the wrong place (probably the IDE did it)
Wrong
import {useEffect} from "react/cjs/react.production.min";OK
import React, {useEffect} from 'react';
Got the same error when there is an incompatible react version applied in the runtime. Had to change react and react-dom versions in the main package.json file.
"dependencies": { "react": "^17.0.2", "react-dom": "^17.0.2", ... }
I had the same error, but when using rollup. I solved it by moving react and react-dom to peerDependencies in package.json
"peerDependencies": { "react": "^17.0.2", "react-dom": "^17.0.2" },