Hi,hopefully you're all keeping well!

I'm finding my way around Sketchup quicker than I thought, although new to CAD in general. I am having an issue that I cant seem to get around? (please see attachment)

I have what is two trapezoids of matching faces, aligned using the centre point function along the blue axis. It looks like a pointed edge that should be able to be rounded but instead, (from the error message) it is only 'one side'. Is there anyway to remedy this?

Any input most appreciated.