I had the same problem with Ubuntu 20.04 LTS and this solved the problem for me: https://help.ubuntu.com/community/ResizeEncryptedPartitions

The main steps are:

0. Boot into live USB

1. Decrypt your file system.

sudo cryptsetup luksOpen /dev/sda5 crypt1

2. Get the live CD to recognize (activate) your LVM.

sudo vgscan --mknodes
sudo vgchange -ay

3. Resize the Crypt.

sudo cryptsetup resize crypt1

4. Resize the (LVM) Physical Volume.

sudo pvresize /dev/mapper/crypt1

5. Resize your root (LVM) Logical Volume.

5.1. Unlock the (LVM) Physical Volume.

sudo pvchange -x y /dev/mapper/crypt1

5.2. Resize the (LVM) Physical Volume.

sudo lvresize -l +4G /dev/ubuntu-vg/

Note: Change the +4G to the amount of space you are adding, or +100%FREE to use all of the available space.

5.3. Re-lock the physical volume.

sudo pvchange -x n /dev/mapper/crypt1

6. Resize the filesystem.

sudo e2fsck -f /dev/mapper/ubuntu--vg-root
sudo resize2fs -p /dev/mapper/ubuntu--vg-root

7. Reboot

I had the same problem with Ubuntu 20.04 LTS and this solved the problem for me: https://help.ubuntu.com/community/ResizeEncryptedPartitions

The main steps are:

1. Boot into live USB

2. Decrypt your file system.

   sudo cryptsetup luksOpen /dev/sda5 crypt1
   

3. Get the live CD to recognize (activate) your LVM.

   sudo vgscan --mknodes
   sudo vgchange -ay
   

4. Resize the Crypt.

   sudo cryptsetup resize crypt1
   

5. Resize the (LVM) Physical Volume.

   sudo pvresize /dev/mapper/crypt1
   

6. Resize your root (LVM) Logical Volume.

   1. Unlock the (LVM) Physical Volume.

      sudo pvchange -x y /dev/mapper/crypt1
      

   2. Resize the (LVM) Physical Volume.

      sudo lvresize -l +4G /dev/ubuntu-vg/
      

   Note: Change the +4G to the amount of space you are adding, or +100%FREE to use all of the available space.

   3. Re-lock the physical volume.

      sudo pvchange -x n /dev/mapper/crypt1
      

7. Resize the filesystem.

   sudo e2fsck -f /dev/mapper/ubuntu--vg-root
   sudo resize2fs -p /dev/mapper/ubuntu--vg-root
   

8. Reboot