I have a subscription to Disney+ that I made through the actual website. When I try and login on my roku it wants me to subscribe again. I don't know how to access the account I already have without having to resubscribe
I saw this myself some time ago where one of my Rokus would allow me to login or subscribe while the other one would only allow me to subscribe. I didn't really look into it, but I thought it was odd. Make sure your software is up to date - Settings->System->System update->Check now.
If that doesn't help, try removing the channel from the Home Screen, reboot/restart (Settings->System->System restart), and then add the channel again.
Vague memory from when I first set Disney+ up here (or maybe this was a different channel altogether)...
Try to actually play some program on the channel. It may give you the login option then.