How to fix the kernelstub.Drive : ERROR Could not find a block device for the a partition. This is a critical error and we cannot continue AND Can't update kernel and initramfs in pop_os

This applies to computers with NVMe disks

For some reason, at one point my pop_os stopped working and updating as expected. The following error appeared: 

kernelstub.Drive : ERROR Could not find a block device for the a partition. This is a critical error and we cannot continue.

The source of the problem is that pop_os has at least four partitions: /boot/efi, /recovery, / and swap.

The problem is that the /boot/efi partition is not mounted. 

To fix it you should look for the identification of the boot/efi partition

From a terminal, run: 

lsblk -f

You should see something like this: 

and you will see NOT MOUNTED the /boot/efi, so that should be blank.

To fix this, edit the fstab as follows

sudo gedit /etc/fstab

and add this line

# new for efi

PARTUUID=<copy here your UUID of the nvme0n1p1>  /boot/efi  vfat  umask=0077  0  0

save, close and mount

sudo mount -a

or restart, and that is, fixed!