Comment 2 for bug 1801744

This is caused by insufficient disk space on /boot partition where kernels are stored, excerpts from the attachments:

update-initramfs: Generating /boot/initrd.img-4.4.0-139-generic
cat: write error: No space left on device

Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda2 241965 241595 0 100% /boot

Sometimes, running "sudo apt-get autoremove" will clean up old kernels and allow subsequent upgrade attempts to succeed. In some situations manual intervention is needed, see more about that at https://help.ubuntu.com/community/RemoveOldKernels.