Are you on an Intel Mac or M1?
There are specific instructions for M1 here:
Or have you already been through that?
It looks like there is a bit of a history of this issue happening on both Mac and Windows over the years.
It seems the general theme is that it will do it if there isn’t enough BUS power being supplied to the interface which is odd as it’s the only way you can power it as far as I can see in the manual (are you definitely on the UR22 and not the UR22 MkII?):
Apple mentions the generic issue here:
Sweetwater provides info here for getting Macs and interfaces talking:
https://www.sweetwater.com/sweetcare/articles/resolving-audio-interface-issues-on-mac/
Macs also have loads of permissions barriers now too – have you checked some of this information as well about granting the microphone input access?
Some other people mention making sure the interface is plugged in on boot up or just trying another port:
https://www.reddit.com/r/audioengineering/comments/4sz8dr/ur22_usb_light_flashing_ur22_light_blinking/
https://www.facebook.com/Steinberg/posts/helloi-bought-the-ur-22-interface-for-my-mac-a-couple-months-ago-it-has-worked-p/10153963885216045/
Addionally, the manual has troubleshooting steps on page 13:
Sorry you have been having issues – hopefully with some searching through these you can solve the problem.
BTW Sweetwater has a guide for DAW optimisation on a MAC. It’s for Catalina but it would probably still apply for Big Sur too:
https://www.sweetwater.com/sweetcare/articles/macos-10-15-catalina-optimization-guide-for-recording/