Try this:
If you don't have the Roku mobile app, install it and see if you can log into your Roku account on it. If you have it already, but it's signed into a different Roku account, try signing into the same account as you're using for your "smart" 🙄 home devices. For me, once I signed into the mobile app with the same Roku account that my "smart" devices are linked to I was able to sign in on the smart home app without even needing to enter a password.
Roku Mobile App - Free for iOS® or Android™ | Roku
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