Your map isn't tall enough. You have the map display set to 480 pixels tall, but the layer is only 200 pixels tall. There aren't enough tiles to fill the screen, so some places that you try to put tiles don't have any memory reserved to hold the tiles. Specifically, you will notice that if you move the pointer into the top half of the screen, the tiles coordinates are negative. If you try to put a tile in a negative location, you'll get an error. To fix the problem just change the height of your map to be at least 480 (the height of the display) and then hit the Update Map button.
