Did you just subscribe to a Plex subscription? If so, tldr: restart your Plex server then your client(s) in that order

I got a status code 400 after I’d added the Plex pass thingy to download some stuff to the iPad. I guess it takes a moment or two for everything to catch up, if you’re in a rush though restart your Plex server and then your client in that order so that everything realises you have a pass and it’ll get your download option working