I found what was wrong but not why it happened.
/etc/fstab was BLANK and the filesystem was read only. I copied the fstab file from the image and everything seems fine.
Then I installed plexserver without issue. The only thing I can think of is that I had a manual entry on fstab to load a samba share. Now I’m mounting it through diepi-config and I left fstab as it was.