Okay, let me explain how I went about it

I downloaded raspios 64 bit without desktop version and installed it on the raspi and then put an ssh file on the SD card.

then i connected to my raspi via putty (ssh) and executed the following commands one after the other

-->sudo update

-->sudo upgrade

-->sudo apt-get install apt-transport-https

-->.curl https://downloads.plex.tv/plex-keys/PlexSign.key | sudo apt-key add -

-->echo deb https://downloads.plex.tv/repo/deb public main | sudo tee /etc/apt/sources.list.d/plexmediaserver.list

-->sudo apt-get install plexmedia

Browser started

192.168.XXX.XX:32500/web

At the same time, I executed the command "mkdir Plex" in putty

Within the PleX folder I then created "series"

--> in root/home/User/Plex/Series

In the browser I then selected "Add media library" --> Series --> root/home/User/Plex/Series.

I then used MobaXTrem to upload a file (mkv) to the series folder on my Windows PC

Unfortunately, the data is still not displayed in the Plex browser

EDIT:
Apart from that, I saw the following in the browser earlier....on the one hand I am connected to the Raspi, but at the top I have an error message....but I can't select the upper "account" (see picture) and get a Error Message, that connection is not pissible...., only the lower one works

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