gpg-agent can't be reached
Answering myself in case someone else is stuck on this.
On Ubuntu 16.04, the default versions are:
# gpg --version
gpg (GnuPG) 1.4.20
and
# gpg-agent --version
gpg-agent (GnuPG) 2.1.11
They are not compatible. It is mandatory to use GPG2:
# gpg2 --version
gpg (GnuPG) 2.1.11
The solution is to install gpgv2
:
sudo apt install gpgv2
This is because gpg-agent that comes with Ubuntu is version 2 and it tries to connect to gpg with version 2. But Ubuntu has gpg version 1 installed.