I believe I ended up mixing up permissions at /etc/ssl directories tree as the last modification was made on 18th November and a day after I could not get my PostgreSQL to work.
Try adding postgres
user to the group ssl-cert
Run the below code to fix your issue:
# > It happened to me and it turned out that I removed erroneously the postgres user from "ssl-cert" group, set it back with
sudo gpasswd -a postgres ssl-cert
# Fixed ownership and mode
sudo chown root:ssl-cert /etc/ssl/private/ssl-cert-snakeoil.key
sudo chmod 740 /etc/ssl/private/ssl-cert-snakeoil.key
# now postgresql starts! (and install command doesn't fail anymore)
sudo /etc/init.d/postgresql start
courtsey to GabLeRoux