I have been trying to removed FTA Puerto Rico channel from the manage subscription menu with no success.
Many of the Subscription channels require you to cancel the subscription first before you can remove it from the Roku device. Head over to https://www.roku.com/ and sign in to your dashboard account and cancel the subscription under "Manage Subscriptions". Then try again to remove FTA Puerto Rico.
Generally, only the free channels/apps can be removed from the device simply using the * button. Or those in which you pay the provider directly.
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If you have already canceled and still having trouble just removing the channel/app tile, go to Streaming Channels on the device home screen, search for the FTA channel/app, select it, and look for option "Remove Channel".