Edit: Do you own a RPi 1 or Zero or something like that? These devices are not supported by Plex. The output “main/binary-arm/Packages” tells me it has to be such a device.
Edit 2: Please read Error: package architecture (arm) does not match system (armhf) Your apt has to try to download armhf and not arm packages.
We also have a small summary of the special steps required on Raspberry Pi 2-4 and similar devices.
If you still are unsuccessful to update Plex please post much more details about your system (hardware, OS,…).
