Hi Sakura

Thank you for posting in the Lucid Community! There are a couple of ways to allow an arrow to be placed on top of a shape the first option is to turn off Line connections. The line connections feature automatically locks onto an anchor point of a shape when turned off lines will act similar to flowchart shapes and can be piled. To turn your line connections off navigate to File > Page Settings > Line connections. This will affect all future lines placed on your document and can be toggled on and off.

If you would prefer to keep the settings as they are you can also place the line on another layer which will keep these two shapes separate from one another. To move a line or shape to another layer select the shape/icon/line and right click on your mouse or trackpad. One the popup menu has opened navigate to Move to Layer>New Layer. This will move your line to the layer above your shape!Hope this helps! Please feel free to post any followup questions on this thread.