We have written the following code to sync contacts between two gmail ids, such that if a new contact is being added to one email id and the Google Apps Script is run, the new contacts get synced to the other email id too:

const CLIENT_ID = 'client_id'; const CLIENT_SECRET = 'client_secret';

// Replace with the initial refresh tokens for both users
const USER1_REFRESH_TOKEN = 'Reftoken1';
const USER2_REFRESH_TOKEN = 'Reftoken2';

function syncContacts() {
  try {
    // Get fresh access tokens for both users
    const user1AccessToken = refreshAccessToken(USER1_REFRESH_TOKEN);
    const user2AccessToken = refreshAccessToken(USER2_REFRESH_TOKEN);

    // Fetch contacts from both accounts
    const user1Contacts = fetchContacts(user1AccessToken);
    const user2Contacts = fetchContacts(user2AccessToken);

    // Sync contacts between accounts
    addUniqueContacts(user1Contacts, user2AccessToken);
    addUniqueContacts(user2Contacts, user1AccessToken);

    Logger.log('Contacts synced successfully!');
  } catch (error) {
    Logger.log(`Error during sync: ${error.message}`);
  }
}

function fetchContacts(accessToken) {
  const url = 'https://people.googleapis.com/v1/people/me/connections?pageSize=2000&personFields=names,emailAddresses,phoneNumbers';
  const options = {
    method: 'get',
    headers: {
      Authorization: 'Bearer ' + accessToken,
    },
    muteHttpExceptions: true,
  };

  try {
    const response = UrlFetchApp.fetch(url, options);
    const result = JSON.parse(response.getContentText());

    if (response.getResponseCode() === 200 && result.connections) {
      Logger.log(`Fetched ${result.connections.length} contacts.`);
      return result;
    } else {
      Logger.log('Error fetching contacts: ' + response.getContentText());
      return null;
    }
  } catch (error) {
    Logger.log('Error fetching contacts: ' + error.message);
    return null;
  }
}


function addUniqueContacts(sourceAccessToken, targetAccessToken) {
  const sourceContacts = fetchContacts(sourceAccessToken);
  const targetContacts = fetchContacts(targetAccessToken);

  if (!sourceContacts || !targetContacts) {
    Logger.log('Failed to fetch contacts. Aborting synchronization.');
    return;
  }

  const targetPhoneNumbers = new Set(
    targetContacts.connections.flatMap(contact =>
      contact.phoneNumbers?.map(phone => formatPhoneNumber(phone.value)) || []
    )
  );

  const uniqueContacts = sourceContacts.connections.filter(contact =>
    (contact.phoneNumbers || []).some(phone =>
      !targetPhoneNumbers.has(formatPhoneNumber(phone.value))
    )
  );

  Logger.log(`Found ${uniqueContacts.length} unique contacts.`);

  uniqueContacts.forEach(contact => {
    const sanitizedContact = sanitizeContact(contact); // Sanitize before creating
    try {
      createContact(targetAccessToken, sanitizedContact);
    } catch (error) {
      Logger.log('Error adding unique contact: ' + error.message);
    }
  });
}

function sanitizeContact(contact) {
  // Remove metadata, resourceName, etag, and other unnecessary fields
  return {
    names: contact.names?.map(name => ({
      displayName: name.displayName,
      givenName: name.givenName,
    })),
    emailAddresses: contact.emailAddresses?.map(email => ({
      value: email.value,
    })),
    phoneNumbers: contact.phoneNumbers?.map(phone => ({
      value: phone.value,
    })),
  };
}

function createContact(accessToken, contact) {
  const url = 'https://people.googleapis.com/v1/people:createContact';
  const options = {
    method: 'post',
    headers: {
      Authorization: 'Bearer ' + accessToken,
    },
    contentType: 'application/json',
    muteHttpExceptions: true,
    payload: JSON.stringify(contact),
  };

  try {
    const response = UrlFetchApp.fetch(url, options);
    const result = JSON.parse(response.getContentText());

    if (response.getResponseCode() === 200) {
      Logger.log('Contact Created: ' + JSON.stringify(result));
    } else {
      Logger.log('Error creating contact: ' + response.getContentText());
    }
  } catch (error) {
    Logger.log('Error creating contact: ' + error.message);
    throw new Error('Failed to create contact.');
  }
}

        function refreshAccessToken(refreshToken) {
  const url = 'https://oauth2.googleapis.com/token';
  const payload = {
    client_id: CLIENT_ID,
    client_secret: CLIENT_SECRET,
    refresh_token: refreshToken,
    grant_type: 'refresh_token',
  };

   const options = {
    method: 'post',
    contentType: 'application/x-www-form-urlencoded',
    payload: Object.keys(payload)
      .map((key) => `${encodeURIComponent(key)}=${encodeURIComponent(payload[key])}`)
      .join('&'),
    muteHttpExceptions: true, // Capture full error details
  };

  try {
    const response = UrlFetchApp.fetch(url, options);
    const result = JSON.parse(response.getContentText());

    // Log the response for debugging
    Logger.log(`Token refresh response: ${JSON.stringify(result)}`); 

     if (result.access_token) {
      Logger.log('Access token refreshed successfully.');
      return result.access_token;
    } else {
      throw new Error(result.error_description || 'Failed to refresh access token.');
    }
  } catch (error) {
    Logger.log(`Error refreshing access token: ${error.message}`);
    throw new Error('Access token refresh failed.');
  }
} 

         function formatPhoneNumber(phone) {
  // Standardize phone numbers for comparison
  return phone.replace(/[^d+]/g, ''); // Removes non-digit and non-'+' characters
}

However, I keep getting this error:

Error fetching contacts: {
  "error": {
    "code": 401,
    "message": "Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.",
    "status": "UNAUTHENTICATED"
  }
}

These are the following ways I have tried trouble shooting:

1. Check if People API is enabled for both email ids.

2. Set up Credentials from one account and share with the other.

3. Set up these following redirect URI’s:
   http://localhost, https://script.google.com/oauthcallback, https://developers.google.com/oauthplayground

4. Have authorised API in Google playground from both id’s and stored correct refresh tokens

I am not sure how to rectify this error. Would be really helpful if someone could guide?