W: Репозиторий 'cdrom: // Xubuntu 16.04 LTS _Xenial Xerus_ - Выпуск [дубликат]

следующая команда в терминале ubuntu выдает ошибку

sudo apt-get update

Это сообщение об ошибке

W: The repository 'cdrom://Xubuntu 16.04 LTS _Xenial Xerus_ - Release 
amd64 (20160420.1) xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore 
potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user 
configuration details.
E: Failed to fetch cdrom://Xubuntu 16.04 LTS _Xenial Xerus_ - Release 
amd64 (20160420.1)/dists/xenial/main/binary-amd64/Packages  Please use 
apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot 
be used to add new CD-ROMs 
E: Some index files failed to download. They have been ignored, or old 
ones used instead.