Just because you can reach twocows.com, it doesn't mean you can get the repository your system is expecting.
There are a few things that might be the case:
- You put the repo information in wrong. This is likely if it's a new one, but not if it's worked before.
- The repo has been removed. This is likely if you're running an old version of the software or the platform (Ubuntu, for example, has removed the repositories for 9.04).
- The repo doesn't have information on your version of Ubuntu. This one is the most likely. During the upgrade, Ubuntu updates your repository paths. If the repository doesn't have a branch for your version, then it will error out. If it's stopping your upgrade, deactivate the repository for the time being. Once you've upgraded to 12.10, you can reactivate the repository, making sure it still points to the 12.04 ("precise") branch. (Note: That doesn't guarantee that software will still work. If it depends on an API that has been broken in 12.10, then it may not work as expected. In which case, you'll have to wait until Two Cows puts out a "quantal" branch.)