我正在尝试按照此页面http://wiki.ros.org/melodic/Installation/Ubuntu中的说明在 ubuntu 18.04 上安装 ros melodic ，当我使用此命令时，sudo apt install ros-melodic-desktop-full我收到以下消息
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-melodic-desktop-full : Depends: ros-melodic-desktop but it is not going to be installed
Depends: ros-melodic-perception but it is not going to be installed
Depends: ros-melodic-simulators but it is not going to be installed
Depends: ros-melodic-urdf-sim-tutorial but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
我试图安装 ros-melodic-desktop 但它不断给我依赖错误消息。任何帮助，请。