I have a "new" 2021 Samsung Smart TV and just hooked up my Roku to it. (I like Roku menu better than Samsung). Roku is now stuck on National Parks Screensaver, and I'm not able to use it/delete it...
First, try pressing the Home button on your Roku remote to exit screensaver. (should wake up your device and put you on Roku Home screen).
If unsuccessful, restart your Roku device. (unplug device and plug back in to force a reboot). Suggested to use the power adapter that came with the device and plug into wall outlet rather than TV USB power port.
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After your Roku reboots, you can change your Screensaver if you think this was the issue. (Settings/Theme/Screensavers). However, sounds like your Roku may have just gone to sleep or entered low power mode which requires the press of Home button to wake up.
Feel free to post an update or if you need more help.