How to create an archive for Google Docs?

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I’m currently using the following script but encountering an error that states:

unexpected error in UrlFetchApp.fetch()

function createArchive() {
    var oauthSettings = UrlFetchApp.addOAuthService("docsAPI");
    oauthSettings.setAccessTokenUrl("https://www.example.com/oauth/access");
    oauthSettings.setRequestTokenUrl("https://www.example.com/oauth/request?scope=https://docs.example.com/api/");
    oauthSettings.setAuthorizationUrl("https://www.example.com/oauth/authorize");
    oauthSettings.setConsumerKey(consumerKey);
    oauthSettings.setConsumerSecret(consumerSecret);

    var requestOptions = {
        "method": "POST",
        "headers": { "GData-Version": "3.0" },
        "oAuthServiceName": "docsAPI",
        "payload": generatePayload(),
        "oAuthUseToken": "always"
    };

    var apiEndpoint = 'https://docs.example.com/api/archive';
    var response = UrlFetchApp.fetch(apiEndpoint, requestOptions);
}

function generatePayload() {
    var xmlString = '<?xml version="1.0" encoding="UTF-8"?>' +
        '<atom:entry xmlns="http://www.w3.org/2005/Atom" xmlns:docs="http://schemas.example.com/docs/2007">' +
        '<docs:archiveConversion source="application/vnd.google-apps.document" target="application/msword"/>' +
        '<docs:archiveConversion source="application/vnd.google-apps.spreadsheet" target="text/csv"/>' +
        '<docs:archiveConversion source="application/pdf" target="application/pdf"/>' +
        '</atom:entry>';
    Logger.log(xmlString);
    return xmlString;
}

If there’s an alternative method to achieve this, please share it with me as soon as possible. Thank you!