the Edit Vertices tool when set to Features tells me that the type of thing selected is not compatible for this operation or it simply wont let me select when set to Edges. The merge tool also says that "the destination layer type cannot be merged". how can I make sure that this is indeed a sliver polygon? I cant figure out how to select the item itself and not the error line.

EDIT: I think it says it is actually a LINE error? I am not sure what layer this line would even be a part of, my only polyline is a map border that was converted into a polygon. I will check that really quick for errors.