I'm guessing that the error message reads something like
"The files could not be imported because they could not be read or because the destination folder is not writeable"
This usually means that you do not have write permissions on the folder you are importing to - probably the permissions got changed when you installed Sierra.
Right click the folder, choose Get Info, and make sure permissions are set to both read and write.
(not a Mac user myself, so I can't be more specific)