Why am I being charged $51 a month from Roku and I try to cancel my subscription that I never signed up for but am being charged for and it says I have no subscriptions or purchase history!
Call Roku Customer Service: (816) 272-8106 choose option 1 (Accounts/Billing)
Otherwise, you may be the victim of a scam/fraud, and you'll need to contact your bank.
Scammers can name their bogus charges anything they like. Don't assume that a charge that says it comes from Roku necessarily really is from Roku.
Log into your account at https://my.roku.com/subscriptions (oops) https://my.roku.com/account/subscriptions using the email address shown via Roku menu path Settings > System > About. You can cancel any subscriptions made through Roku there. If you have multiple Rokus and any of them shows a different email address, repeat this check on those Roku accounts as well.
If you see no subscriptions listed on the Roku site(s), challenge the unrecognized charges with the financial institution through which the charges are arriving. They have the resources to trace them back to their actual origin.