If you click through the items until something shows up in the right hand pane. That will tell you the setting that has been changed by a group policy. It will also tell you the policy name so you can remove it.
Alternatively, if you open up CMD and type gpresult /r and press OK it will list all applied policies to the PC then you can review each policy to see which is applying the disabled permission.
Check in there for any policy changing the setting which disables you from using regedit.
Alternatively, if you open up CMD and type gpresult /r and press OK it will list all applied policies to the PC then you can review each policy to see which is applying the disabled permission.