If you haven’t tried it yet, the first recourse should be to restart the system. These lists are maintained by a Launch Services daemon that caches them in system memory. If the cache has somehow gotten corrupted, that can cause your problem. Restarting will force the daemons to regenerate their caches and might fix the problem.
If the restart fails, possibly the files where OS X saves the data have been damaged. The file location and format has changed (I think maybe as of OS X 10.12?), so that will affect where you need to look. Your post says OS X 10.11, so I’ll emphasize the older system.
Earlier releases of OS X kept the list of recent files for SketchUp 2017 in
Note that is your personal Library folder, not the system one. Finder hides it by default, so if you don’t see it you will have to either use Finder’s Go->Go to Folder item or to make it permanently visible, select your home folder and in View->Show View Options check the box for “Show Library Folder”.
You can try deleting or renaming that file while SketchUp is not running (if you do so while SketchUp is running, the system will generally overwrite the plist from its cache when SketchUp closes and restore the previous values). You may also have to log off and then back on to get the change to stick, as the system caches user preferences values in memory during a login session. Don’t delete or rename other .plist files, as that will kill off all of your other preferences too!