What Sass means by "@function rules may not contain style rules."

When writing a Sass function, you may run into an error message that may be confusing:

Copy@function rules may not contain style rules.

What Sass is trying to tell you is that a line in your Sass function is computing a result that is neither assigned to a variable nor returned.

Keep in mind that all functions in Sass return something, Sass does not mutate objects.