Refer: https://techtips.easycloudsolutions.com/2018/05/15/ubuntu-update-error-waiting-for-unattended-upgr-to-exit/
Stop the automatic updater.
$ sudo dpkg-reconfigure -plow unattended-upgrades
At the first prompt, choose not to download and install updates.
Make a reboot. Make sure any packages in an unclean state are installed correctly.
$ sudo dpkg --configure -a
Get your system up-top-date.
$ sudo apt update && sudo apt -f install && sudo apt full-upgrade
Turn the automatic updater back on, now that the blockage is cleared.
$ sudo dpkg-reconfigure -plow unattended-upgrades
Select the package unattended-upgrades again.