You're using the minified versions of React and ReactDOM, which are only meant to be used when you deploy your application out into the wild. They're a lot smaller and faster, but this comes at the cost of a lot of the nice development features being stripped out. This includes all the error messages, which is why they seem so confusing to you!
To fix this, you should make sure you're using the uncompressed, development versions of React and ReactDOM. While generally you should be using NPM and a module bundler like Webpack to install them into your project (or a scaffolding tool like create-react-app), if you just want to download the files you can do so here:
Once you've replaced the scripts, you'll be able to see what the actual error message is.