If the manual above sounds too technical, there is an alternative. The modding community realized the pain of the downgrade patch and created an all-in-one installer.
PES 2006 Downgrade Patch eFootball PES 2021 (often called the PES2006 Classic Patch Pes 2006 Downgrade Patch Fix
Are you running the or CPY version of PES 2021 so I can provide more specific troubleshooting steps? If the manual above sounds too technical, there