install postgresql showing errors.

Hello,
I am trying to deploy my rails app on digital ocean. and am following this article https://gorails.com/deploy/ubuntu/18.04
but when i am trying to run this command sudo apt-get install postgresql postgresql-contrib libpq-dev
it is showing the following errors.

Reading package lists... Done
Building dependency tree       
Reading state information... Done
libpq-dev is already the newest version (11.2-1.pgdg16.04+1).
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:
 postgresql : Depends: postgresql-11 but it is not going to be installed
 postgresql-contrib : Depends: postgresql-contrib-11
E: Unable to correct problems, you have held broken packages.

What is the problem an what should i do.

[I am workign with my new user, not in root user]

Thank you