XMLHttpRequest error in flutter web [Enabling CORS AWS API gateway]

My server was using nginx so I solved the problem by adding the following two lines to the server block of the sites-enabled config file for my API server:

add_header Access-Control-Allow-Origin "*";
add_header Access-Control-Allow-Methods "GET, HEAD";

My app only uses GET and HEAD so you may need to add other methods depending on your situation.

See also: How to Enable CORS in Apache and Nginx?

this worked for me, I added the below header on the lambda function

return {
    statusCode: 200,
     headers: {
  "Access-Control-Allow-Origin": "*", // Required for CORS support to work
  "Access-Control-Allow-Credentials": true, // Required for cookies, authorization headers with HTTPS
  "Access-Control-Allow-Headers": "Origin,Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token,locale",
  "Access-Control-Allow-Methods": "POST, OPTIONS"
},
    body: JSON.stringify(item)
};