I have just created one SPFx web part with react framework. I tried testing through SharePoint online workbench (https://demo.sharepoint.com/sites/test/_layouts/15/workbench.aspx).
I am getting an error. Chrome browser developer tool shows below error:
Failed to load resource: the server responded with a status of 404 (Not Found)
https://localhost:4321/node_modules/react-dom/dist/react-dom.js
https://localhost:4321/node_modules/react/dist/react.js
Based on above mentioned error, I noticed the "node_modules/react/" does not contains "dist" folder at all.
gulp serve
is running properly and try hard refresh (Ctrl + F5) on SharePoint workbench once after gulp serve is running. which version of SPFx generator you are using?