Roku themselves does not create channels for other providers. Discovery has to make a channel for Roku devices. And then, they have to make it available in a specific geographic area.
In this case, there is a Discovery+ channel for Roku devices. But if it's not in your device's channel store, that means Discovery hasn't made it available in your country. You have to ask Discovery to change that. Roku is not permitted to alter the geographical settings for 3rd party channels.
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