Unable to log in to Crunchyroll on The Roku Channel

Yeah, I ran into the same issue. The Crunchyroll on the Roku Channel is actually separate from the regular Crunchyroll app it doesn’t let you log into an existing account. You’d have to start a new subscription through Roku, which is kind of frustrating.
If you already have a Crunchyroll subscription, just install the official Crunchyroll app from the Roku Channel Store (not the one inside Roku Channel). That version lets you log in normally and use your existing account. Bit confusing, but it works!