cPanel's PostgreSQL support is still a bit rudimentary, and as of 11.25 RELEASE there still is one major bug with the installation. After following the PostgreSQL installation steps on in the documentation, existing users still will not have the ability to create databases. You will see the following in the error log indicating this:

ERROR: role "user" doesn't exist

cPanel will also die without an error in the interface when attempting to create a database. To fix this, run the following from a root shell:

# cd /var/cpanel/users && for x in *; do su -c "createuser -S -D -R $x" postgres; done