If you are running a version of Chargent later than 2.92, license keys are not needed to use Payment Request and Payment Console in the Sandbox.
ChargentSFA.TChargentStaticHelper.GatewayException: Payment Request feature is not activated!
This error is caused by a custom URL for the Payment Request site, which does not include the Sandbox instance number (eg cs11, cs40, etc.).
Go to the Chargent Settings and change the Sites page from something like https://custom.secure.force.com/ to something like https://custom.cs80.secure.force.com/. It should be recognized as a sandbox and the error should go away.