This appears to be related to free space on /boot.
I deleted all kernel 4.15 files from /boot and then update-initramfs was
able to complete.
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https://bugs.launchpad.net/bugs/1899666
Tit
Screen cap of failure
** Attachment added: "Screen Shot 2020-10-13 at 11.41.30 AM.png"
https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1899666/+attachment/5421772/+files/Screen%20Shot%202020-10-13%20at%2011.41.30%20AM.png
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