With SIP you are prevented from modifying things in the /System tree, as well as /usr, /bin, /sbin, and any of the pre-installed apps that come with Sierra. I'm guessing that the problem is something called SIP (System Integrity Protection). I also found this post: Re: Custom Profile Issue

Further down in the instructions, when signed into the 'root' user, I delete the Keychain folder contents then instead of while logged in as the default profile user. The few things I did differently were running the Mac version of CCleaner with everything checked excluding 'Custom Files and Folders'. Re-adding the folder to the dock fixes it, but that's not desirable in the field! On different machines it could also show a giant question mark. I got everything to work, except the Downloads folder in the dock shows the folder icon with the little red circle, as if the user doesn't have rights to it, yet I can add/remove files no problem. In my set up, I open some preinstalled programs, like LibreOffice and Firefox, add some bookmarks and install some extensions. These instructions worked flawlessly in Mavericks, but I'm also having a few quirks in Sierra. I followed this guide written for Mavericks to El Capitan, but performed on macOS Sierra:

Change profile icon for mac osx