Jose got a new 4K TV and a Roku Ultra. But the Roku Ultra is getting an error for resolution. Leo says Jose needs an HDMI 2.1 cable to support the 4K stream. You don't need a monster cable or anything like that. Just one that's HDMI 2.1 compatible. Leo recommends MonoPrice.com. It's around $12.
But shouldn't the cable that came with the Roku be compatible? Leo says it should be. Try resetting the Ultra and see if it has the latest firmware. If the problem persists, then it's likely a bad Roku unit, and you should take it back and exchange it for another one.