To create a swap partition after an Ubuntu installation create an empty partition that should have no holes. You can then format this partition with
sudo mkswap /dev/sdx
replacing sdx with your partition. Mount this partition as swap with
sudo swapon -U UUID
where UUID is that of your /dev/sdx as read from this
Bind your new swap in /etc/fstab by adding this line:
UUID=xxx none swap sw 0 0
If you want to use your swap for hibernating then you need to update the UUID in /etc/initramfs-tools/conf.d/resume with this content RESUME=UUID=xxx. Don’t forget to $ sudo update-initramfs -u.