what needed to mount mac filesystem? (ubuntu live cd on macbook pro)

Hello!
I have successfully booted a MacBook Pro with a Ubuntu 18.04 Live CD. When I try to mount the internal mac HD in the file manager app, I get a lengthy error message: "Unable to access location. Error mounting /dev/sda2 at /media/ubuntu/Macintosh HD: wrong fs type, bad option, bad superblock on /dev/sda2, missing codepage or helper program, or other error."

I thought it would mount readonly without problem? Is this a sign that the HD is encrypted or do I just need to install something?

Thanks in advance!