90-updates-available exited with return code 255

Enter console

Ctrl+Alt+F1

Complete your update and reboot your system

sudo dpkg --configure -a

If the above gives errors of broken packages, execute below command

sudo apt-get install -f
sudo dpkg --force-all -P pkgName
sudo dpkg --force-all -P linux-image-extra-3.xxxxxxx-generic

sudo apt-get autoremove
sudo apt-get autoremove --purge