Using VirtualBox, I'll often give Ubuntu a certain disk size and only realize later on that it isn't using the full partition size for the disk.
Run these commands to extend the root filesystem to use the full disk:
lvextend -l +100%FREE /dev/ubuntu-vg/ubuntu-lv
resize2fs /dev/mapper/ubuntu–vg-ubuntu–lv
df -h
For more details, see: https://packetpushers.net/blog/ubuntu-extend-your-default-lvm-space/