Reading package lists... Done Building dependency tree... Done Reading state information... Done E: Unable to locate package yum

The error message you're encountering is from a Linux package manager, typically APT (Advanced Package Tool) which is used by Ubuntu and its derivatives, while `yum` is commonly associated with the RPM (Red Hat Package Manager) system found in CentOS, Fedora, or RHEL. The error "E: Unable to locate package yum" indicates that the system is looking for a package called `yum`, but it can't find it in the repository.

Here's what's happening:
- `Reading package lists... Done` means APT is gathering information about available packages.
- `Building dependency tree... Done` follows by constructing the list of dependencies for the requested package(s).
- `Reading state information... Done` checks if the necessary packages are installed or if there are any issues.

The problem arises because `yum` is not a package managed by APT; it's a separate package manager used mainly on Red Hat-based systems. To resolve this issue on an Ubuntu system:

1. **Identify the appropriate package manager:** Since you're using APT, you need to install software through it, not `yum`.
2. **Install missing package (if applicable):** If you're trying to install something that should be installed via APT, make sure you're using apt-get or apt instead.
3. **Update the repositories:** Run `sudo apt update` to ensure your package list is up-to-date.
4. **Install a Red Hat package manager (optional):** If you actually need `yum`, consider installing `apt-transport-https` and `epel-release` for accessing the Extra Packages for Enterprise Linux (EPEL) repository, which may have `yum`. Then use `sudo apt install yum` or `sudo yum install yum`.