From Case Study: Swapoff Cannot Allocate Memory.
If processes reserve more memory than the sum of the swap area and (part of the) RAM, and your system is configured not to over-commit memory, the allocation fails. This can happen even while you have plenty of RAM free and are not using any pages on the swap area.
This article lets you know how to enter the emergency mode, which will essentially mount the system in a read only format. You will need to remount the system in rw to be able to edit your fstab and remove the invalid swap line
You can remount ...
The original kernel (4.13) of 17.10 and later solved the slow swap file issue. Performance was the sole reason to change. Before then, a swap partition was superior.
Which kernel do you use? Find it with cat /proc/version then click  and add that useful information. If < 4.13, stay with a swap partition.
If your motivation is enabling hibernation,...
Firstly, I would suggest a lighter desktop than GNOME, as one of our PLUG members had problems with a 12 camera Zoneminder install last week. I'd try Ubuntu MATE*, Xubuntu, or Lubuntu (all 18.04.3 versions).
He found success with Ubuntu MATE after failing with standard Ubuntu.
Let the default install set up a swap file, then monitor it (you know how) with ...
For a 1Tb HDD, I'd recommend the following partitioning:
/ <== primary partition, 10gb root partition with **boot flag**
/usr <== logical partition, 24gb read-only user data
/var <== logical partition, 26gb variable files
swap <== logical partition, 11gb used when psysical RAM memory is full
/tmp <== logical partition, 12gb temporary ...
Honestly, I wouldn't overengineer this situation. Of course, there're different scenarios and for each of them, you might want a different size of swap, but for normal needs you might have at home, you can stick to this recommendation by Red Hat: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/installation_guide/sect-disk-...
This is a bit better than swapon -s as it provides human friendly size units such as:
NAME TYPE SIZE USED PRIO
/dev/dm-1 partition 7.5G 563.8M -2
man swap says:
Display swap usage summary by device. Equivalent to "cat /proc/swaps". This output format is DEPRECATED in favour of --show that ...