401 Unauthorize error when making POST requests

Question

I'm using SharePoint's REST APIs to build a simple application that displays all items of a list and includes a form to create a new list item. All users are able to view the items, but only a few users can't create a new list item because of a 401 unauthorize error. Trying to retrieve the Form Digest Value also results in a 401 unauthorize error because it's a POST request. Here are the functions to create a new list item and to retrieve the form digest value:

const createItem = async (data, url) => {
    const digestVal = await getFormDigestValue().then(data => data.d.GetContextWebInformation.FormDigestValue);

    return $.ajax({
        url,
        type: "POST",
        headers: {
            // Accept header: Specifies the format for response data from the server.
            "Accept": "application/json;odata=verbose",
            //Content-Type header: Specifies the format of the data that the client is sending to the server
            "Content-Type": "application/json;odata=verbose",
            // X-RequestDigest header: When you send a POST request, it must include the form digest value in X-RequestDigest header
            "X-RequestDigest": digestVal
        },
        data: JSON.stringify(data)
    })
}

const getFormDigestValue = () => {
    return $.ajax({
        url: 'https://example.com/_api/contextinfo',
        method: 'POST',
        headers: { Accept: 'application/json; odata=verbose' }
    });
}