Karl9 you mean the issue is you (the admin) being unable to select the "Members" virtual group in the Permissions page dropdown because it's off screen ? That's the only thing I can guess from your screenshot.
Are you able to resize your browser so there's enough vertical space for the whole dropdown to pop under ?
That dropdown component is not very well designed as it lacks the ability to turn into a scrollable component when the vertical space is not enough... Also there's a "dumb" check that opens the dropdown upwards if there's not enough space under it, even if there's even less space above it ? I had to apply some CSS in some of my extensions to make it work better.
If you're completely unable to access the button, you could use the browser inspect tool to add the following styles to the dropdown html tag:
height: 100px; /* anything that can fit the viewport */
scroll-y: scroll; /* will add a scrollbar */