Comment 2 for bug 1893495

The error in your log is:

sh: printf: I/O error

It is unclear to me how this error occurred. It appears that you were attempting an upgrade from xenial to bionic, but not using the release upgrader, and that stdout/stdin have somehow been disconnected from any tty, resulting in read/write failures for the maintainer scripts. I am not sure this is a bug that can be fixed in shim-signed.