Try troubleshooting the Safari .plist (preferences can become corrupted)
Open the Finder. From the Finder menu bar click Go > Go to Folder
Type this exactly as you see it here:
Move the com.apple.Safari.plist file to the Desktop.
Quit and relaunch Safari.
Try adding a new bookmark of bookmark folder. If you can, then move the .plist file to the Trash. If not, move it back to the Preferences folder.
If that didn't help, open System Preferences > iCloud
Deselect the box next to Bookmarks, then reselect it.
Quit and relaunch Safari. Try a bookmark again.
edited by: cs