For CompactLogix and ControlLogix controllers, your RSlogix5000 or Studio5000 version must match your controllers firmware. For example, to use Studio5000 v30, your controller must have firmware V30 installed. If you have a controller with firmware V20 installed, you must install RSLogix5000 V20.
The L6x series of processors cannot be flashed above V20. Therefore, the latest version of software you can use with your processor is RSLogix5000 V20.04. Your processor must also have firmware version 20. If you are working with an existing application that was installed on firmware v17 for example, you will need to install RSlogix5000 v17 or you can upgrade the program in RSlogix v20, flash up the processor to v20, and reload the program to the processor.