This is apparently a known bug on some versions of Chrome. Follow the instructions below to fix it:

If you are downloading XKit from somewhere other than the Google Web Store:

Don’t. You are probably using an outdated link, or an unauthorized mirror. Do not install extensions for Chrome from anywhere other than the Web Store, they might be broken or modified in ways that might compromise your security/privacy.

If you are downloading XKit from the Google Web Store:

Apparently, a workaround for this is to go in “Incognito Mode” (Ctrl-Shift-N) and install the extension there. You can try this, or right-click and download it, and then go to your browser settings > Extensions and drag-drop the file there.