Swap: From partition to file, now get "no matching swap device is available"

That line is in /etc/initramfs-tools/conf.d/resume so try deleting it.

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The file in question, /etc/initramfs-tools/conf.d/resume, hasn't even existed in an Ubuntu LTS release since 14.04. Deleting the one line it contained on an 18.04 system will never cause a problem. This is a great example of why a reinstall is preferable to an upgrade, sometimes old things get left which can later cause problems.

As for those who think blkid will produce anything at all for a swapfile, you are wrong, and the answer which suggest using blkid is non-responsive to the stated problem. Reread the question, there is no longer a swap partition.

As a comment suggested, a UUID for the swapfile may be produced with swaplabel, but it is left as an exercise for the reader to see if using that swapfile UUID in a configuration file which is no longer part of the Ubuntu system will actually fix anything, or just result in another UUID lookup failure.

Step 1:

$ sudo blkid

It will list get UUID of each device or partition on your system. Copy the UUID of swap partition.

Step 2:

$ sudo vim /etc/initramfs-tools/conf.d/resume

Replace the available UUID with paste the UUID which you just copied and save it.

Step 3:

$ sudo update-initramfs -u