I cant aply the gradient

/* BACKGROUND BUILDINGS - “bb” stands for “background building” */

.bb1a {
width: 70%;
height: 10%;
background-color: linear-gradient(
–building-color1,
–window-color1
);
}

Challenge: Learn CSS Variables by Building a City Skyline - Step 40

Link to the challenge:

The step asks you to apply the linear-gradient to the background property, NOT background-color.

However, I can verify that even applying the linear-gradient to the background property does not work either. Completely stuck at this step.