When you load your app, you're presumably going to the / route. When using a browser to visit a page, a GET request is sent. Since the / route only allows POST requests, Flask returns an error.

POST requests are used when data needs to be sent to the server. See the corresponding MDN page for more information

Since no processing of user data is done in the main function, remove methods=["POST"] from its decorator. This will set the allowed methods to GET. (see Flask Docs)