Errors: grub-efi-amd64-signed & shim-signed

pI need to fix the following problems

I do not have experience in linux, so please possible give me simply explanations and the code to type in command line and fix the problems, please!

I checked other threads and I found similar problems but non the same.

Question

manolis@manolis-300-0xx:~$ sudo apt-get upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
  netplan.io
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] Y
Setting up grub-efi-amd64-signed (1.93.8+2.02-2ubuntu8.7) ...
Installing for x86_64-efi platform.
Could not prepare Boot variable: No space left on device
grub-install: error: efibootmgr failed to register the boot entry: Input/output error.
dpkg: error processing package grub-efi-amd64-signed (--configure):
 installed grub-efi-amd64-signed package post-installation script subprocess returned error exit status 1
dpkg: dependency problems prevent processing triggers for shim-signed:
 shim-signed depends on grub-efi-amd64-signed; however:
  Package grub-efi-amd64-signed is not configured yet.

dpkg: error processing package shim-signed (--configure):
 dependency problems - leaving triggers unprocessed
Errors were encountered while processing:
 grub-efi-amd64-signed
 shim-signed
E: Sub-process /usr/bin/dpkg returned an error code (1)

Backgournd Informations

NAME="Ubuntu"
VERSION="18.04.1 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.1 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

manolis@manolis-300-0xx:~$ echo ${BASH_VERSION%%[^0-9.]*}
4.4.19

manolis@manolis-300-0xx:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            1,9G     0  1,9G   0% /dev
tmpfs           387M  2,0M  385M   1% /run
/dev/sda2       916G  207G  663G  24% /
tmpfs           1,9G   61M  1,9G   4% /dev/shm
tmpfs           5,0M  4,0K  5,0M   1% /run/lock
tmpfs           1,9G     0  1,9G   0% /sys/fs/cgroup
/dev/loop0      3,8M  3,8M     0 100% /snap/gnome-system-monitor/51
/dev/loop2       88M   88M     0 100% /snap/core/5742
/dev/loop1       15M   15M     0 100% /snap/gnome-logs/45
/dev/loop4      141M  141M     0 100% /snap/gnome-3-26-1604/70
/dev/loop6       13M   13M     0 100% /snap/gnome-characters/117
/dev/loop7       13M   13M     0 100% /snap/gnome-characters/124
/dev/loop9      140M  140M     0 100% /snap/gnome-3-26-1604/64
/dev/loop10      35M   35M     0 100% /snap/gtk-common-themes/319
/dev/loop8      2,3M  2,3M     0 100% /snap/gnome-calculator/238
/dev/loop5      128K  128K     0 100% /snap/tree/15
/dev/loop11     3,8M  3,8M     0 100% /snap/gnome-system-monitor/54
/dev/loop12      13M   13M     0 100% /snap/gnome-characters/139
/dev/loop13      88M   88M     0 100% /snap/core/5662
/dev/loop3      2,3M  2,3M     0 100% /snap/gnome-calculator/260
/dev/loop14      43M   43M     0 100% /snap/gtk-common-themes/701
/dev/loop15      15M   15M     0 100% /snap/gnome-logs/43
/dev/loop16      15M   15M     0 100% /snap/gnome-logs/40
/dev/loop17     141M  141M     0 100% /snap/gnome-3-26-1604/74
/dev/loop18      88M   88M     0 100% /snap/core/5548
/dev/loop19     2,3M  2,3M     0 100% /snap/gnome-calculator/222
/dev/loop20     3,8M  3,8M     0 100% /snap/gnome-system-monitor/57
/dev/sda1       511M  6,1M  505M   2% /boot/efi
tmpfs           387M   40K  387M   1% /run/user/1000
/dev/sdb1       1,9T  108G  1,8T   6% /media/manolis/Maxtor
tmpfs           387M   16K  387M   1% /run/user/120

manolis@manolis-300-0xx:~$ sudo apt install udo
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following NEW packages will be installed:
  udo
0 upgraded, 1 newly installed, 0 to remove and 1 not upgraded.
1 not fully installed or removed.
Need to get 205 kB of archives.
After this operation, 598 kB of additional disk space will be used.
Get:1 http://it.archive.ubuntu.com/ubuntu bionic/universe amd64 udo amd64 6.4.1-4 [205 kB]
Fetched 205 kB in 0s (1.520 kB/s)
Selecting previously unselected package udo.
(Reading database ... 168775 files and directories currently installed.)
Preparing to unpack .../archives/udo_6.4.1-4_amd64.deb ...
Unpacking udo (6.4.1-4) ...
Setting up udo (6.4.1-4) ...
Setting up grub-efi-amd64-signed (1.93.8+2.02-2ubuntu8.7) ...
Installing for x86_64-efi platform.
Could not prepare Boot variable: No space left on device
grub-install: error: efibootmgr failed to register the boot entry: Input/output error.
dpkg: error processing package grub-efi-amd64-signed (--configure):
 installed grub-efi-amd64-signed package post-installation script subprocess returned error exit status 1
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
dpkg: dependency problems prevent processing triggers for shim-signed:
 shim-signed depends on grub-efi-amd64-signed; however:
  Package grub-efi-amd64-signed is not configured yet.

dpkg: error processing package shim-signed (--configure):
 dependency problems - leaving triggers unprocessed
Errors were encountered while processing:
 grub-efi-amd64-signed
 shim-signed
E: Sub-process /usr/bin/dpkg returned an error code (1)

Why I have all of these "/dev/loop" ??? How can I clean and increase the space: "Could not prepare Boot variable: No space left on device"

Is correct to do, or dangerous:

rm -r /dev/loop*

All the best!