I tried existing solutions on 18.04 and found it doesn't work with a 'nothing to do' output. Some further searching had similar-but-slightly-different steps and needing to merge a few different bits-and-pieces from other solutions on the web to make work.

Setup: Hyper-V; VHDX file as hard disk; Ubuntu-18.04

Steps:

1. Expand the VDHX file via Hyper-V as mentioned in existing solutions and then inside the VM:

2. fdisk -l

   See which partition is the current Ubuntu setup - should be obvious based on size (in my case was sda3)

3. growpart /dev/sda 3

   Note the space as mentioned.

4. pvresize /dev/sda3

   This is the step which isn't mentioned in a lot of places; its the intemediate step that allows the logical volume extension step work.

5. lvextend -l +100%FREE /dev/ubuntu-vg/ubuntu-lv

   The /dev/ part can seen in the fdisk output from step 1.

6. resize2fs /dev/mapper/ubuntu--vg-ubuntu--lv

   After the prep above, this step now works. Takes a couple of moments and afterwards can verify with df -h that the partition is expanded.

src = https://superuser.com/questions/1716141/how-to-expand-ubuntu-20-04-lts-filesystem-volume-on-hyper-v