There was an issue earlier today where some customers encountered an error when trying to checkout using PayPal Express. A fix has since been pushed however, so this issue should now be resolved.
If your customers are still experiencing difficulties with checking out via PayPal on your store, you can get them to try and complete their purchase via incognito mode, a different browser, or by using a different device altogether. If the issues still persist, you can contact our live support team via the Help Center here so that we can further troubleshoot and investigate the situation in more detail from our end.