“MAD did not answer INIT command”

Hello Marcindulak,
Today I have started from scratch and followed
Front-end Installation — OpenNebula 5.4.15 documentation for installation. When I run

wget -q -O- http://downloads.opennebula.org/repo/repo.key | apt-key add - command. I got gpg: no valid OpenPGP data found error. I have created a new thread for this issue. Please provide your suggestions.