*the code seems to be working fine, or at least the window is showing the gradient, but I keep getting informed that I should (add a background property”)

Tell us what’s happening:
Describe your issue in detail here.

  **Your code so far**
.bb1a {
background: linear-gradient(--building-color1, --window-color1);
width: 70%;
height: 10%;
background-color: var(--building-color1);
}

Link to the challenge:

Here you use variable to set color, right?
You need to use variables with proper syntax to set color inside linear-gradient also.

And I think you have some typos:

we need double hyphen before word building here:

--