Why can't I mount a swap partition?

Check if swap partition is set up correctly

Execute the command sudo fdisk -l. This will list all your partitions and their partition types. In Ubuntu, a swap partition will be listed similar to below:

/dev/sdb4            7295        8268     7811072   82  Linux swap / Solaris

If you don't see the last 2 values 82 and Linux Swap, then it is not setup as swap partition.

Setup a swap partition

Ensure that the partition you are going to use (such as /dev/sdb4) is not a real partition with real data before turning it in to swap partition since all your data will be lost.

The following steps will set up a swap partition:

sudo swapoff -a

Replace /dev/sdb4 with the correct partition name if different:

sudo /sbin/mkswap /dev/sdb4

Enable the swap partition you just set up:

sudo swapon -a

For detailed help, refer to https://help.ubuntu.com/community/SwapFaq

Simple: you don't have an sdb4 partition! Your swap appears to be on /dev/mapper/cryptswap1. It looks like you setup cryptswap, so you need to fix your fstab to point to the new swap partition, preferably by UUID, not by device name.