Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
Some channels are geo-restricted to certain countries. If you're not in the US, that might be the reason. I can't say for sure, but your set is certainly capable of having Pluto.
Roku does not make the decision to geo-restrict channels. This is done by the channel providers, in this case Pluto TV. You would have to ask Pluto to make the channel available outside the US. Because of licensing agreements for the content they offer, they might not be able to do so.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.