Often the Ubuntu upgrades will fail fue to a no space error. The only solution is to remove old kernels manually.
The first step is to show which kernel you are currently running, as you do not want to delete this one.
sudo su uname -r
So change to the boot directory for simplicity and list all existing kernels on the system.
cd /boot ls -al
Then you can remove the older kernels…
apt remove linux-image-4.15.0-99-genericapt remove linux-image-4.15.0-99-generic
… and clean up.
apt --purge autoremove
You can retry the upgrade
apt full upgrade