January 27, 2016
ACA: Ceiling Grid Boundary Selection and Resource Manager Error
"But wait!" you exclaim. "That Polyline looks closed to me." It may very well looked closed, but when drawing the Polyline, merely snapping the last point to the first point does not a closed Polyline make. Next time, stifle the urge to add one more point on top of the first point, and use the Close command option, by selecting it on the Command Line, or by typing C and then pressing the ENTER key.
BONUS TIP: You can double-click that Polyline to initiate the PEDIT command on that Polyline. Personally, I would get rid of that last vertex and then close the Polyline, but that is not necessary for the Polyline to be a valid Ceiling Grid boundary.
Curiously, the Set boundary option of the CEILINGGRIDCLIP command provides an informative message at the Command line, rather than the non-informative Error dialog the CEILINGGRIDADD command gives, when selecting an open Polyline as the boundary.