Create new Cluster for current data in /main.

Step by step.

(1) sudo systemctl stop postgresql

(2) edit postgresql.conf

data_directory = '/var/lib/postgresql/12/main3'

(3) sudo -u postgres /usr/lib/postgresql/12/bin/initdb -D /var/lib/postgresql/12/main3

(4) sudo cd /var/lib/postgresql/12/

(5) sudo rm -Rf main3

(6) sudo cp -a main main3

(7) sudo systemctl start postgresql