If you're using the Railway plugin inside ChatGPT, it connects to your Railway account through OAuth, so an invalid authentication error means that connection needs to be made again. Remove the Railway plugin in ChatGPT, reinstall it from the Railway plugin listing, connect your Railway account when prompted, and start a new chat (plugin docs).

If instead you connected your ChatGPT subscription to the Railway Agent, go to Account Settings, then Agents and click Re-Authenticate on the ChatGPT card, then approve the new activation code. Requests on that subscription fail until this is done (troubleshooting).