I can't properly compile this level. When I do, I receive the following message:

"This map requires one or more textures that can't be found. Replace with default textures?"

If I select "Yes", my sky is filled with stone tiles.
When I attempt to make the sky brighter or darker by pressing 1 or 2 the following error message pops up:

Malfunction W1501
Empty pointer in bright_lights: sky_cube_level.blue+=10

There seems to be something missing in this workshop level.
could somebody please help me?

Here's the example code for the workshop 13 level
////////////////////////////////////////////////////////////////////

var video_mode = 7; // 800x600 pixels

////////////////////////////////////////////////////////////////////

function bright_lights()
{
sky_cube_level.blue += 10;
sky_cube_level.green += 10;
sky_cube_level.red += 10;
}

function dim_lights()
{
sky_cube_level.blue -= 10;
sky_cube_level.green -= 10;
sky_cube_level.red -= 10;
}

function main()
{
on_1 = bright_lights;
on_2 = dim_lights;

level_load ("work13.wmb");
while (1)
{
if (key_w == ON)
{
camera.tilt += 1;
}
if (key_s == ON)
{
camera.tilt -= 1;
}
if (key_a == ON)
{
camera.pan += 1;
}
if (key_d == ON)
{
camera.pan -= 1;
}
if (key_lastpressed == 19) // if [R] is pressed
{
camera.pan = 0;
camera.tilt = 0;
sky_cube_level.blue = 128; // medium brightness
sky_cube_level.green = 128;
sky_cube_level.red = 128;
}
wait (1);
}
}