You can access the payroll menu or Employees tab by logging in on your QuickBooks Online account via web browser: qbo.intuit.com. Also, make sure that you were given an access for payroll by the primary administrator. You can check with them by going to Employees tab and then on Payroll Settings select Manage users.
Once you've confirmed that you were given an access for payroll and you've used a web browser, you can check your browser settings if third party cookies is enabled.
Here's how to enable third party cookies on Chrome:
From the menu, click Settings.
Scroll to the bottom of the settings screen and click Show advanced settings.
Under the Privacy header, click the Content Settings button.
Under the Cookies section, ensure the Block third-party cookies and site data option is not ticked.