This can be caused by a couple of things.
First Login to PayPal go to Profile > Email addresses
Make sure all of your emails are confirmed.
Next
Login to PayPal > Profile > Website Payment Preferences.
On the Website Payment Preferences Page there will be a section labeled "PayPal Optional" make sure that is turned on.
If the above fails.
Click your PayPal button on your site look at the email address at the top of the page. Make sure the email address is spelled correctly and it is an email that is on your PayPal account.
One of the above should resolve the issue.
If not go to
https://www.paypal.com/mts and file a support ticket.
Hope that helps.
next time if you provide a link to the button I would be able to possibly give you a little bit of a better answer.