The version of shim-signed that you were upgrading to is the release version of shim-signed in 18.04. This version of shim-signed cannot be supported; it is 4 years old, and missing all security updates to shim that have been released in the past 4 years. It will quite simply become unbootable at a future date.
The issue you're reporting is one that was fixed at some point in the history of bionic. You should have bionic-updates enabled when upgrading between releases. The simplest way to achieve this is to use do-release-upgrader or update-manager to manage the upgrade, which appears not to have been done here. ** Changed in: shim-signed (Ubuntu) Status: New => Fix Released -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1961641 Title: package shim-signed 1.34.9+13-0ubuntu2 failed to install/upgrade: installed shim-signed package post-installation script subprocess returned error exit status 1 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/shim-signed/+bug/1961641/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs