If you're using the component contact enhanced, it has a bug which generate that paypal error because it messes with the email hidden input fields of the PayPal form of HikaShop and it shouldn't.
In that case, remove it or contact contact enhanced support to fix the problem.
If you don't use that component, make sure that you entered a valid paypal email address in the options of the plugin ( you can confirm that with paypal's support ).
If that still is not working, please provide a link to your website so that we can look at the issue.