If the restart does not resolve the problem, then try the following.
1. Remove the LBP6000 from Printers & Scanners.
2. Open Finder and select Go > Go to Folder. Then type (or copy) the following and press the Go button.
/Library/Printer/Canon
Here you will see the CUPSCAPT folder. Trash this folder.
3. Still in Finder, use the Go > Go to Folder and type (or copy) the following and press the Go button.
/Library/LaunchAgents
Trash the file "jp.co.canon.CUPSCAPT.BG.plist"
4. Last action in Finder is to use the Go > Go to Folder and type (or copy) the following and press the Go button.
/Library/Caches
Here you will see the Canon folder which you can Trash.
5. With the above steps complete, empty the Trash and restart the Mac.
6. After you log back in, run the CAPT v3.67 installer again and then add the LBP6000 in Printers & Scanners.