Problem on installing Ansible on 18.04 LTS

What I did:

sudo apt-add-repository ppa:ansible/ansible ; sudo apt update ; sudo apt install ansible

Error message:

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:
 ansible : Depends: python-paramiko but it is not installable
E: Unable to correct problems, you have held broken packages.

Description:
As you see, the problem is with the python-paramiko package. I tried to install the required package but it hasn't solved the problem.

In addition, none of the following related questions solved the problem.

• Unable to install ansible due to python dependency on Ubuntu 18.04
• Ansible from ppa won't install python-related packages

Thanks in advance.