Some extensions cannot be extended for more than a few years. By default, WHMCS prevents domain registration or renewal for more than 10 years. But for some extensions these numbers are different.
For example, some extensions can not be extended for more than 6 years, and when a customer extends a domain for more than this number of years, an error occurs and we have to do manual operations to return these operations and financial operations.
There should be an option to change this number for specific extensions.