Mac mini: Messages app is blocked by information window

Hi. I have a new Mac mini. When I open the Messages app, the Messages window is overlayed (and effectively blocked) by a 'Shared with You' information window. I cannot get rid of this window. The buttons to acknowledge this window appear to be off the bottom of my screen. My display settings are fine. I have attempted to change the display settings in order to reduce the size of the blocking window, to no avail. How can I get rid of this 'Shared with you Window', which has rendered Messages on my Mac mini unusable? Thanks

Edited to say: Problem now solved. I was able to set the dock to ' Automatically hide and show the dock' and just about get my cursor to part of the OK button to delete the offending window (which cannot be resized or moved). Very poor work by whoever at Apple created it!