You may encounter this error when creating sloped glazing or a curtain element with a small opening.
Error: Cannot create curtain panel in opening. The opening is probably too small.
Issue: A horizontal and vertical curtain grid are not meeting exactly at a point, creating a small triangular panel, which cannot support mullions.
Solution: Cancel the placement of the mullion. When dividing the panels in a sloped glazing, it is best to start with the placement of horizontal curtain grids and then snap vertical grids to the horizontal grid.