Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response, with a Chrome extension

Consider:

I'm working on a Google Chrome devtools panels extension. In it I have:

const toDbBtn = document.getElementById("toDbBtn");

toDbBtn.addEventListener("click", async () => {
  const dbName = document.getElementById("textBox").value;

  // const result = await chrome.storage.local.get(["yourKey"]);

  const storedJSON = await chrome.storage.local.get(["yourKey"]);
  console.log("Result from storage:", storedJSON);

  if (storedJSON.yourKey) {

    const parsedArray = JSON.parse(storedJSON.yourKey);
    console.log("Parsed Array:", parsedArray);
    const partArray = parsedArray.slice(0, 30);
    console.log("partArray", partArray);
    const data = {
      dbName: dbName,
      savedArray: partArray,
    };

    const postable = JSON.stringify(data);
    console.log("postable", postable);
    // http://localhost:8888/.netlify/functions//updateCollection.js

    const response = await fetch(
      "http://localhost:8888/.netlify/functions/updateCollection",
      {
        method: "POST",
        headers: {
          "Content-Type": "application/json",
          // "Access-Control-Allow-Origin": "*",
          // "Access-Control-Allow-Methods": "POST",
          // "Access-Control-Allow-Headers": "Content-Type",
        },
        body: postable,
      }
    );

    const result = await response.json();
    console.log("Response from server:", result);
  } else {
    console.log("Value not found in storage for key: yourKey");
  }
});

I'm planning to send some data to a Netlify function, and so I decided to test locally using the "NTL DEV" command.

My target function is:

const headers = {
  "Cache-Control": "no-cache",
  "Cross-Origin-Opener-Policy": "unsafe-none",
  "Cross-Origin-Embedder-Policy": "unsafe-none",
  "Access-Control-Allow-Origin": "*",
  "Access-Control-Allow-Private-Network": "true",
  "Access-Control-Allow-Headers": "Access-Control-Request-Private-Network",
  "Access-Control-Allow-Methods": "OPTIONS,POST,GET,HEAD,QUERY,query",
};

exports.handler = async function (event, context) {

  console.log("context: ", context);
  console.log("event: ", event);

  if (event.httpMethod !== "OPTIONS" && event.httpMethod !== "POST") {
    console.log("event.httpMethod: ", event.httpMethod);
    return {
      statusCode: 405,
      body: JSON.stringify({ message: "Method Not Allowed" }),
    };
  } else if (event.httpMethod === "OPTIONS") {
    return {
      statusCode: 204,
      headers: headers,
      body: null,
    };

  }
  if (event.httpMethod === "POST") {
    console.log("event.httpMethod: ", event.httpMethod);
    console.log("event.body: ", event.body);
    const data = JSON.parse(event.body);
    console.log("data: ", data);
  }

I'm getting the error in the title. What am I doing wrong?