When Ubuntu first get installed on a machine, the available disk space may be smaller than the actual disk space.

To expand the space to the full disk size, we need to do two things:

sudo lvextend -l +100%FREE /dev/mapper/ubuntu--vg-ubuntu--lv

sudo resize2fs /dev/mapper/ubuntu--vg-ubuntu--lv

Checkout this question on askubuntu.com for more info.