@Dylan I apologize if I have made anything confusing with my ramblings. To clarify, I installed HexOS following the standard install guide in February and it installed fine. I then used the HexOS Plex app install option, and it worked fine, but I never used it. Now just a couple days ago I try to use Plex and noticed it said there was an update available, but that update didn't work, so I uninstalled Plex through HexOS (which was successful) and tried reinstalling it through HexOS (which failed with the permissions error). I have never gone into TrueNAS for anything or modified any permissions. So I'm not sure how things got into this state, but I fully agree that this sounds like some kind of permissions issue.
Since I have never done anything inside of TrueNAS, do you know if there is a guide somewhere regarding setting permissions? And what would I be looking for on the applications dataset/folder? You suggest making sure the application has write access, would that be HexOS having write permissions? Like, am I looking to give HexOS write permissions or am I looking to give Plex write permissions?
Sorry, I just want to make sure I know what I'm looking for before I go into TrueNAS so I don't mess anything up, because the NAS itself has more space used than I have backup storage available. Thank you for your help so far, I really appreciate it!