"Certain files in this folder cannot be shared due to a takedown request with the DMCA"
I interpret that as he has some copyrighted files in his private folder which he is trying to make shared/public.
Perhaps dropbox has previously gotten a takedown request for those files (when shared by some other user), and to comply with the request is preventing anyone else from sharing the files in the future.
"Certain files in this folder cannot be shared due to a takedown request with the DMCA"
I interpret that as he has some copyrighted files in his private folder which he is trying to make shared/public. Perhaps dropbox has previously gotten a takedown request for those files (when shared by some other user), and to comply with the request is preventing anyone else from sharing the files in the future.