I also have this bug. Both on Ubuntu (gnome) and Ubuntu Mate. I even posted it on Ubuntu Mate's community forum, they just instruct me to delete the keys via terminal. No solution or reason why it doesn't work on the GUI part.
And checking on this report, bug exist since January 2017? Will this get even fixed? Details: Description: Ubuntu 18.04.1 LTS Release: 18.04 software-properties-gtk: Installed: 0.96.24.32.5 Candidate: 0.96.24.32.5 Version table: *** 0.96.24.32.5 500 500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages 500 http://archive.ubuntu.com/ubuntu bionic-updates/main i386 Packages 100 /var/lib/dpkg/status 0.96.24.32.1 500 500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages 500 http://archive.ubuntu.com/ubuntu bionic/main i386 Packages -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1656100 Title: Unable to remove signing keys using gnome-software-properties To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu-gnome/+bug/1656100/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs