You need to add the exact url route (absolute path) you used in your code, which handles the authorization callback functions in the Backend

Add this url http://localhost:3000/auth/linkedin/callback endpoint in the LinkedIn developer under Auth section
https://developer.linkedin.com/

Ref the MS docs: https://learn.microsoft.com/en-us/linkedin/shared/authentication/authorization-code-flow?tabs=HTTPS1#step-1-configure-your-application