Parse PostCSS mixins error - a term expected

Hi!
have a WebStorm project with postCSS & plugins.
postCSS config:
```js
const path = require('path');

const env = process.env.NODE_ENV;
const plugins = [
    require('postcss-cssnext')({
        // browsers: ['Chrome >= 64', 'Firefox >= 58', 'Safari >= 11', 'iOS >= 10.13', 'Edge >= 41'],
        features: {
            autoprefixer: true,
        },
        cascade: false,
    }),
    require('postcss-utilities')({
        ie8: false,
        centerMethod: 'flexbox',
    }),
    require('postcss-mixins')({
        mixinsDir: path.resolve('./src/common/assets/css/mixins'),
    }),
];

if (env === 'production') {
    plugins.push(require('cssnano'));
}

module.exports = {
    plugins: plugins,
};
```

and have a mixin in size.pcss:
```css
@define-mixin size $size {
    width: $size;
    height: $size;
}
```
project compiles successfully but there is an error in code before symbol $ - Error: a term expected.

can you help me what the error is about, please!