generic warning of "system error" immediately after booting

I'll check again what happens trying a further booting

ProblemType: Package
DistroRelease: Ubuntu 16.04
Package: shim-signed 1.19~16.04.1+0.8-0ubuntu2
ProcVersionSignature: Ubuntu 4.8.0-41.44~16.04.1-generic 4.8.17
Uname: Linux 4.8.0-41-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
BootEFIContents:
 fw
 fwupx64.efi
Date: Mon Mar 13 11:17:31 2017
Df:
 File system 1K-blocchi Usati Disponib. Uso% Montato su
 /dev/sda1 477010936 8186976 444570112 2% /
 /dev/sdb1 7813088 2664224 5148864 35% /cdrom
 udev 1732660 0 1732660 0% /dev
 tmpfs 349652 5680 343972 2% /run
EFIBootMgr: Error: command ['efibootmgr', '-v'] failed with exit code 2: efibootmgr: EFI variables are not supported on this system.
ErrorMessage: subprocess installed post-installation script returned error exit status 1
RelatedPackageVersions:
 dpkg 1.18.4ubuntu1.1
 apt 1.2.19
SourcePackage: shim-signed
Title: package shim-signed 1.19~16.04.1+0.8-0ubuntu2 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1
UpgradeStatus: No upgrade log present (probably fresh install)