If you grew the disk while the VM was powered on, you need to reboot it before continuing. Go to “Manager” > “System” > “Storage” and you will see an “Extend partitions” button. Click on it:
Then, reboot the machine for the changes to have effect.
After the reboot, go back to “Manager” > “System” > “Storage” and will see a “Extend filesystem” button. Click on it:
Now your filesystem has more free space.