After checking the following: 1) my Roku device model number was eligible for screen mirroring, 2) checking my iOS was 12.3 or higher, 3) insuring the Roku and my iphone were on the same 2.4 Ghz wifi, 4) verifying the Airplay setting was set to On, I still could not screen mirror my iphone 8 to my Roku Express device.
I was able to move the Roku to a newer TV (had been using a 2008 Samsung flatscreen) and was then able to do screen mirroring. One other thing I did, I sent back to my older TV and changed the wifi signal to the faster 5Ghz wifi signal and that enabled screen mirroring on my 2008 Samsung. Hope this helps someone.
