Are you getting any specific when trying to open it?
Do you have other files labeled , etc., in the same folder? 7zip question - Noobs - LaunchBox Community Forums File: Dream_Knockout_1.7z ...
Note: If the 7z file contains game ROMs or emulated software, you may need to map the extracted files within an emulator tool. To help you better, could you tell me: Are you getting any specific when trying to open it