Re: Error. This extension cannot be installed, since it conflicts with an existing one. To install this extension, please remove the extension 'GuideGuide' which has been installed in", then install again.

Hi,

Have you ever uninstalled Photoshop CC and installed Photoshop CC 2014, or uninstalled Photoshop CC 2014 then installed Photoshop CC?

If that, please re-install you uninstalled Photoshop, and remove GuideGuide from Extension Manager before uninstall.

Another option is remove extension manager database file, then launch Extension Manager to install it.

C:\ProgramData\Adobe\Extension Manager CC\Configuration\DB\ExMan.db (Windows)

/Library/Application Support/Adobe/Extension Manager CC/Configuration/DB/ExMan.db (MAC)

But please notice that all your previously installed extensions will not be shown in Extension Manager after you removed database file.