Can I screen mirror my Google pixel 3 A xl screen to my Roku ultra device?
Have you tried?
Otherwise, if by "cast" you mean screen mirror (using Miracast), then you wont be able to do so until you root your phone and make some modifications:
https://techlector.com/how-to-connect-google-pixel-3a-pixel-3a-xl-to-tv/
https://howtotechnaija.com/connect-google-pixel-3-and-pixel-3-xl-to-tv/
(You can thank Google for disabling/hiding the industry-standard Miracast functionality in its later Android versions and devices in favor of its proprietary-and-incompatible-with-Miracast Chromecast functionality...)