I'm following the official guide to install ROS Melodic.
And, in step 1.3, I got something similar to the following, by 3 of the 4 alternative paths:
sudo apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
Executing: /tmp/apt-key-gpghome.Esk2aHFoQ2/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com:80 --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
gpg: key F42ED6FBAB17C654: public key "Open Robotics <[email protected]>" imported
gpg: Total number processed: 1
gpg: imported: 1
gpg: invalid key resource URL '/etc/apt/trusted.gpg.d/home:hpcoder1.gpg'
gpg: keyblock resource '(null)': General error
gpg: key 7721F63BD38B4796: 2 signatures not checked due to missing keys
gpg: key 5606C8D03F9E5FB8: 1 signature not checked due to a missing key
gpg: key 3B4FE6ACC0B21F32: 3 signatures not checked due to missing keys
gpg: key D94AA3F0EFE21092: 3 signatures not checked due to missing keys
gpg: key 871920D1991BC93C: 1 signature not checked due to a missing key
gpg: Total number processed: 10
gpg: skipped new keys: 10
In the other one I got:
sudo apt-key adv --keyserver 'hkp://pgp.mit.edu:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
Executing: /tmp/apt-key-gpghome.ABFSlwnvVU/gpg.1.sh --keyserver hkp://pgp.mit.edu:80 --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
gpg: keyserver receive failed: No data