Can't delete a folder in Ubuntu?

Well… are you sure is not owned by root? Have you tried getting into the terminal and remove it with “sudo”?

sudo rm -r oldfolder

Which folder is it? (You might not want to delete it :)

Have you checked who the owner is? How was it created? Is it perhaps on an external (read-only?) drive?

It might be possible that you did not have permission to delete folder…for that
u should first give the permission to access by following command as mentioned below
chmod -R 777 <folder-path>
then run command as
rm -f <folder-path>