Q: Digital Camera RAW Compatibility Update 6.18 - will not install on Mac mini

Try rebooting the computer

I have this issue with the Digital Camera RAW Compatibility Update 6.18 also on my iMac (late 2013) running 10.11.2.  I click on Upgrade, it goes gray for a few seconds, and then nothing happens.  I did reboot the computer as suggested, but that did not help.

If you have an update that's stuck, try deleting everything in the /library/updates folder, restarting the app store app, then downloading/installing the update again.  That worked for me with the last update I had trouble with (itunes).  The only consequence is that it's now listed three times in the update history rather than once.

This answer solved this issue for me.

https://origin-discussions-us.apple.com/message/29469040#29469040

Had to restart and install the update again but it at least got in me going in the right direction. The update finally installed and went away.

Hope this helps someone else.