Running PMS on old MacMini that cannot run new version and cannot access server from ROKU or Amazon Fire

It looks like you have set the path of the Plex Server meta data to be the exact same path where you also store your media.
If you delete it, you will lose all your libraries.
Mingling will lead to malfunctioning of Plex.

Move the metadata folder to a location/folder that is not scanned by Plex for media.
Here’s a guide how to do that: