apt - Unable to install postgreSQL 9.6 in Ubuntu 18.04

I'm trying to install postgreSQL 9.6 in Ubuntu 18.04 via Ubuntu Software Centre and type from terminal

sudo apt-get install postgresql-9.6

according to official documentation https://www.postgresql.org/download/linux/ubuntu/

Create the file /etc/apt/sources.list.d/pgdg.list and add a line for the repository

deb http://apt.postgresql.org/pub/repos/apt/ bionic-pgdg main

Import the repository signing key, and update the package lists
wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc |   sudo apt-key add -
sudo apt-get update

The version to be installaed is 10

Any help?

Thanks in advance.

postgresql:

Installed: (none)
Candidate: 10+191.pgdg18.04+1
Version table:
10+191.pgdg18.04+1 500
500 http://apt.postgresql.org/pub/repos/apt bionic-pgdg/main amd64 Packages
500 http://apt.postgresql.org/pub/repos/apt bionic-pgdg/main i386 Packages
10+190 500
500 http://gr.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
500 http://gr.archive.ubuntu.com/ubuntu bionic/main i386 Packages

If I check where are the postgresql folders, it seems that I have both versions, 9.6 and 10

/usr/lib/postgresql

After few months I had to erase and install from scratch, so in order to install postgresql 9.6 I followed next steps:

Important notes: If you have already installed postgresql 10 and you want 9.6, you need to remove postgresql 10 completely and then manually install postgresql 9.6, so follow method 2.

Method 1

Step 1

sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ `lsb_release -cs`-pgdg main" >> /etc/apt/sources.list.d/pgdg.list'

Step 2

wget -q https://www.postgresql.org/media/keys/ACCC4CF8.asc -O - | sudo apt-key add -

Step 3.

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install postgresql-9.6

Method 2

In order to install postgres 9.6 having already postgres 10 or other version different from 9.6 installed, you need first to uninstall postgresql (any version and file related) completely following the next steps.

sudo apt-get --purge remove postgresql
dpkg -l | grep postgres (to look for postgresfiles in the system)
sudo rm -rf postgresql ... (remove all the files that appeared in the list after running the previous command)

Finally install manually postgreSQL with the next command:

sudo apt-get install postgresql-9.6

I hope it can help somebody that could have the same problem.