Cannot use rosnode command

edit

When I use rosnode command, the following error is shown.

 zhou@zhou-Inspiron-7447:~$ rosnode list

    Command 'rosnode' not found, but can be installed with:

    sudo apt install python-rosnode

Then I run

sudo apt install python-rosnode

The result is as follows:

zhou@zhou-Inspiron-7447:~$ sudo apt install python-rosnode
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:
 python-rosnode : Depends: python-rostopic but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

How can I solve the problem? My PC OS is ubuntu 18.04 LTS, and the version of ROS is Melodic. Thanks.