If you are not an expert user (like me) and have an SMB share working with full permissions (create folders, delete folders, copy files, delete files etc) then you can add your Plex media folders in that share instead of trying to create another separate one. Make a subfolder called ‘Plex’ within the main share, and then the usual Plex media folders within that (‘Films’, ‘Audio’, ‘Recordings’, etc). Thoroughly check that you can manipulate the files within the Plex folder manually, before installing Pex as an app.
For the Plex install, I found the following guide very useful. It is for a whole TrueNAS Scale install, then setting up an SMB share, then finally installing Plex as an app. You will need to pick up at the right point in the guide for the Plex bit but it’s also useful if you need to backtrack and change some parameters of the SMB share to Windows.
I made an adapted version of the guide for my own use - more or less clarifying various steps in the above one - which I’m happy to post if you can’t get on with the original.