Ubuntu is a distribution of Linux. This means it's an OS built around the Linux kernel, the core of the OS. There are many, many distributions, and they all are designed to fit a particular purpose or preference for the user. Linux Mint is a derivative of Ubuntu, which in itself is a derivative of Debian.

Anyway, those errors seem to all be 404. Maybe they're having issues on their end, or your Internet connection is acting up? Try again later.

As previously stated, be aware of what you're adding to your repo list and make sure it's from a reputable source. Also, don't cross-pollinate repos from different distributions in an attempt to get more or newer software, it leads to Bad Things.