The output clearly tells you the repo is still NOT available and gives you instructions how to install the update manually. Please read the update text.
##### ATTENTION #####
Finally, official ARMv7 and ARMv8 Plex packages are available: https://www.plex.tv/media-server-downloads/
The repo on dev2day.de has nearly reached its end of life. Please upgrade to the official packages as soon as possible. This repository will disappear sometime in the future.
The migration is simple and your Plex library will be preserved.
First, remove the dev2day.de package:
sudo apt-get remove plexmediaserver-installer
Then, remove the dev2day.de repo, e.g. with:
sudo rm /etc/apt/sources.list.d/pms.list
Now, download the appropriate package from https://www.plex.tv/media-server-downloads/ and put it on your device, e.g.:
wget https://downloads.plex.tv/plex-media-server-new/1.15.2.793-782228f99/debian/plexmediaserver_1.15.2.793-782228f99_armhf.deb
Finally, install the package, e.g.:
sudo dpkg -i plexmediaserver_1.15.2.793-782228f99_armhf.deb
Also, an official Plex repository should be available soon. Most likely, the repo will still NOT work when you read this message!
https://support.plex.tv/articles/235974187-enable-repository-updating-for-supported-linux-server-distributions/
The following thread has more information: https://forums.plex.tv/t/read-me-first-about-server-armv7-and-armv8-ubuntu-debian/226567/
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