Are you are referring to the Hint page?
https://forum.freecodecamp.org/t/freecodecamp-challenge-guide-create-a-gradual-css-linear-gradient
The correct property for a linear-gradient is background-image, however when using the background shorthand property you are setting it without explicitly specifying the longhand property.
If you look at the computed properties you will see it is set for the background-image property.
Because
<gradient>s belong to the<image>data type, they can only be used where<image>s can be used. For this reason,linear-gradient()won’t work on background-color and other properties that use the<color>data type.