How do I fix the "Unexpected token ' ' doctype ... is not valid JSON" error if I'm trying to fetch data from a web server?

Verify that the server is correctly returning JSON data. Inspect the response headers to ensure the 'Content-Type' header is set to 'application/json'. If the response is HTML, check the server-side code to ensure it's serializing data to JSON format before sending it. If the data is coming from a different source, make sure the request points to a proper API end point.