Here is a circle drawn with the path tool.

Draw a circle on your canvas.

Let's start by using the path tool to draw an object.

Now let's see with which tools we can't edit this path object.  Switch to the eraser tool.  As soon as you point to the circle, or anywhere on your canvas, for that matter a crossed out red circle becomes your pointer.  If you click the circle you drew, this message will be displayed.

Why did you get this error message?  You got this message because the image you drew is a path object.  You cannot edit a path object with the eraser tool.