Installing and removing initramfs-tools-ubuntu-touch removes /usr/sbin/flash-touch-initrd, but leaves the script /etc/initramfs/post-update.d/zz-flash-touch-initrd, which attempts to call /usr/sbin/flash-touch-initrd and fails.
My solution was to purge, not simply remove, initramfs-tools-ubuntu-touch.
I saw this symptom, but for a different reason.
Installing and removing initramfs- tools-ubuntu- touch removes /usr/sbin/ flash-touch- initrd, but leaves the script /etc/initramfs/ post-update. d/zz-flash- touch-initrd, which attempts to call /usr/sbin/ flash-touch- initrd and fails.
My solution was to purge, not simply remove, initramfs- tools-ubuntu- touch.