This means you are already at or above the site or traffic limits you are paying for.

Traffic limits are enforced. When you try to register a new site, we check the average daily traffic from the last 7 days. If you are above this limit, you will not be able to register a new site unless you upgrade.

If you are having a temporary traffic spike, you will need to wait until it has subsided before you will be allowed to register a new site.