This error occurs when the compiler cannot parse the CSS that it scans for custom-property usage. It needs that usage to select property registrations and initial values for the output stylesheet.
How to fix
Correct the malformed CSS in the stylesheet or utility definition that produced it. If the authored CSS is valid, report a small reproduction and the Stillwind and plugin versions.
Syntax errors found while the stylesheet loads usually produce SW13001 or SW13002 instead.