That message really means that the files cannot be copied to the specified destination. You need to check your read/write permissions for the destination folder.
You are on a Mac??
The message is misleading as it is most likely the files can't be written to the destination folder you have chosen.
You need to set read/write permission for the destination folder/drive.
You will find how to do that here: https://support.apple.com/kb/PH18894?locale=en_US&viewlocale=en_US