我一直按照安装步骤进行http://wiki.ros.org/noetic/Installation/Ubuntu但由于以下错误，卡在步骤 1.3 上：
james@James:~$ sudo sh -c 'echo "deb http://packages.ros.org/ros/ubuntu $(lsb_release -sc) main" > /etc/apt/sources.list.d/ros-latest.list'
[sudo] password for james:
james@James:~$ sudo apt install curl # if you haven't already installed curl
Reading package lists... Done
Building dependency tree
Reading state information... Done
**Package curl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'curl' has no installation candidate**
james@James:~$ curl -s https://raw.githubusercontent.com/ros/rosdistro/master/ros.asc | sudo apt-key add -
OK
james@James:~$ sudo apt update
Hit:1 http://id.archive.ubuntu.com/ubuntu focal InRelease
Hit:2 http://id.archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:3 http://id.archive.ubuntu.com/ubuntu focal-backports InRelease
Hit:4 https://dl.google.com/linux/chrome/deb stable InRelease
Hit:5 http://packages.ros.org/ros/ubuntu focal InRelease
Hit:6 http://security.ubuntu.com/ubuntu focal-security InRelease
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
james@James:~$
我尝试使用“sudo apt update”对其进行更新，并且它已更新，但是当我再次尝试“sudo apt install curl”代码时，它仍然无法解决问题。我能做些什么来解决这个问题？（我正在使用 Ubuntu 20.04.6）
更新：
...
All packages are up to date.
james@James:~$ sudo apt install ros-noetic-desktop-full
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
ros-noetic-desktop-full : Depends: ros-noetic-desktop but it is not going to be installed
Depends: ros-noetic-perception but it is not going to be installed
Depends: ros-noetic-simulators but it is not going to be installed
Depends: ros-noetic-urdf-sim-tutorial but it is not going to be installed
E: Unable to correct problems, you have held broken packages.