ReferenceError: RichText is not defined usually comes when you are not importing RichText from prismic-reactjs in your project file where you are rendering the Rich Text field.
Are you importing the RichText from prismic-reactjs library?
import {RichText} from 'prismic-reactjs';
If it still doesn't solve your issue, I'd be happy to dig it more with you.
If your desc is a Rich Text object, it should work with the <RichText render=..... /> method. Can you remove prismic-reactjs library from the pacakge.json file and install it again with the command npm i prismic-reactjs.
However, you can also pass the serializer function in the Rich Text helper function.
If it still doesn't solve your issue, I'd need your source code( Github repo or Zip project) and Prismic repo name. You can send me a private message, though.