Do you have to register your product? Thanks
Your Roku has to be linked to a Roku account in order to work. I don't believe this is referred to a "registering".
Roku devices must be linked to a user account to function. You can't complete the setup without it. But there's no cost, and no requirement to ever pay for anything. One user account can have multiple Roku devices linked, they don't each need their own. But all Roku devices linked to the same account will have the same channels installed. Separate accounts are required if you want different channels on different devices.