This is happening either because the license key is expired or it is not activated on the URL the plugin is installed on. This can sometimes happen if you install the plugin and activate the license key on a temporary URL and then update the site to the final URL.
To resolve the problem, log into your account, click on the License Keys tab, then click on the Manage Sites link for the license key. From that page you will be able to remove URLs for the license.
Next, on your live website, attempt to activate the license again.
Once the proper URL is registered on the license key, you will be able to install the update.
If your license key is expired, you will need to renew it before you can install the update. The status of your license key will be shown on the License Keys tab of your account page.