iMac not providing method to accept screen mirroring from iPhone

I am attempting to mirror my iPhone's screen on my iMac. Both devices have had their iOS updated to the latest versions. I've adjusted the settings on both devices to allow AirPlay transfer. I've restarted both devices. The problem is that when I enable screen mirroring on my iPhone, the screen showing the device selection transmits from my iPhone to my iMac, but there is no way for me to accept the screen mirroring on my iMac. I watched an instructional video for help, and it showed that a pop-up message is supposed to appear in the upper-right corner where screen alerts appear on my iMac, for the user to click on to accept the image transfer. However, no alerts allowing screen mirroring acceptace are appearing at all on my desktop computer. How do I get my iMac to allow the screen mirroring from my iPhone? Thank you for any help you can offer.