While Roku TVs support chromecast, Roku streaming devices do not have built-in Chromecast but supports Apple’s Airplay, the Roku Mobile App, and the DIAL protocol (created by Netflix and YouTube for direct casting). Additionally, it’s compatible with devices supporting Miracast, such as Android and Windows PCs.
Chromecast can’t be used with Roku streaming devices like the Roku stick or Roku ultra, but it can be used on Roku TV as long as it has an HDMI port, Roku TV also supports casting from Android and iOS devices.(See How to screen mirror any device to a Roku)
Read More about Chromecast on Roku: Everything you need to know