As documented (see the section titled The .NET Web Service Interface, under Programming Interfaces), the LicenseKey parameter is used by all Web service methods, and is validated by Currency Server and/or by external validation code (which can optionally be set in the Clients tab of Currency Server Manager).
Valid LicenseKey values are:
* for Currency Server Enterprise Edition
The most common cause for this error is an incorrect license key, for example a software license key used in place of a valid Web service client access key. If you are using Currency Server Enterprise Edition or a trial version of Currency Server, you can simply leave the license key argument empty (unless you configured your own validation code).
If the license key passes basic validation it may still result in a different error, such as "Subscription has expired", "Hit quota exceeded", etc. For a more detailed list of possible conditions, see the section titled Client Validation Modules, under Add-On components.