Starting in Mendix 8 you can build React widgets with the react version that is shipped with Mendix client.
It will not stop you to use React in Mendix 6, The only thing, you have to provide React yourself. Either adding React and React-Dom to the index.html. Or bundle it with the widget. You have to write a wrapper connecting the Mendix Dojo lifecyle to the React, and you have still handle the data subscriptions yourself. All and all not an easy path.
You can get a head start with this NONE supported side project. https://github.com/omnajjar/create-hyper-mendix-widget probably use the variant React (with dojo wrapper) Though I dont know if it will work with 6.
React was introduced with Mendix 8, if you are on Mendix 6 either upgrade to Mendix 8 or build widgets with Dojo