Check the APT sources:

• /etc/apt/sources.list
• official repo in /etc/apt/sources.list.d/official-package-repositories.list

You should see the following:

• cdrom repository (commented or removed post-install)
• Linux Mint repository
• Ubuntu repositories (codename, codename-updates, codename-backports and codename-security)
• The following components for Linux Mint: main upstream import backport
• The following components for Ubuntu: main restricted universe multiverse

Romeo should not be enabled.

Source repos should not be enabled.