One of the reasons for this error can be the fact that one of your customers (or more) is using dedicated IP for his account that is not listed among your license(s). You need to add such IP(s) to the license or move such customers to a shared hosting IP.

When using "Custom (API)" method:
If you use API in order to open website builder, then this error is usually caused by the fact that the license cannot be detected by parameters of API call. The license is searched by 3 different IPs in specified order:

• by FTP host parameter "apiUrl" that you pass among parameters list in API call (only if the parameter "apiUrl" has format of IP address and not of a domain);
• by IP address of parameter "domain" that you pass among parameters list in API call (if it is resolved).
• by outgoing (external) IP address of server that calls website builder API;

If at least one of these cases matches IP(s) in your Site.pro license(s), then the license is considered validated and API returns a valid URL to website builder. Otherwise, the error "License is required" appears.
To fix this issue just make sure that during API call in your case the IP of one of specified 3 cases matches IP(s) in your license.

For On-Premises brand only:

1. Navigate to "My Licenses" page;
2. Open On-Premises brand;
3. Click "Apply" button.

If this action results in error, it means that license synchronization failed and cannot be synchronized with your website builder. If the following did not help to resolve the error message, create ticket in support system and describe this problem.