Roku is currently having some problems with the 4k Spotlight channel on Roku streaming media players and Roku TVs. The app channel unfortunately will not load any content to display. [Update: As of the following day, June 9, the problem seems to have been resolved. No update or restart was needed. Roku also appears to have taken down the support article.]
4k Spotlight highlights movies, TV shows, and other programs that are available in 4k UHD (Ultra HD) definition. However, Roku players need to be 4k compatible to deliver UHD content. Roku players that support 4k include the HDR-capable Roku Ultra HD and Premiere+, as well as the Roku Premiere (non HDR-capable).
Roku is working on fixing the problem and says to try loading the 4k Spotlight channel later. You might also try restarting your Roku if opening the channel doesn’t work. Depending on what the developers do to resolve the issue, you may need to download an update.