I had this errorUnknown error executing apt-key after installing a file into /etc/apt/trusted.gpg.d on a system that had UMASK 077 in /etc/login.defs, so the file was created as 700 and apparently apt-key couldapt-key can not read it because it does not execute as the root user. chmod 644 /etc/apt/trusted.gpg.d/file<file>.asc fixed it.