There is no error here. the Warning is simply a warning that the development server is not intended for production use. (its not been designed with security and load balancing. only intended for testing/dev)
if you were to move it to a server with WSGI capabilities like Apache or Nginx.
you would add one more file to the project flaskapp.wsgi that the server would use in place of your typing “FLASK run” in your dev environment.