Useful or not, from you.
gulp Did you forget to signal async completion?
gulp.task("dist", () => {
  return gulp
    .src(sassDir)
    .pipe(sourcemaps.init())

    .pipe(sass())
    .pipe(autoprefixer())
    .pipe(concat(`${name}.css`))
    .pipe(size())
    .pipe(sourcemaps.write("."))
    .pipe(gulp.dest(distDir))

    .pipe(cssnano())
    .pipe(concat(`${name}.min.css`))
    .pipe(size())
    .pipe(sourcemaps.write("."))
    .pipe(gulp.dest(distDir));
});
$ gulp compile
[09:26:26] Using gulpfile ~/Development/com.github/hiynn-design/gulpfile.js
[09:26:26] Starting 'compile'...
[09:26:26] Starting 'copy-sass'...
[09:26:26] Starting 'copy-indexjs'...
[09:26:26] Starting 'compile-sass'...
[09:26:26] Starting 'dist'...
[09:26:26] Size hiynn-design.css : 135 B
[09:26:26] 'dist' errored after 282 ms
[09:26:26] Error in plugin "gulp-cssnano"
Message:
    /Users/zhangwei/Development/com.github/hiynn-design/hiynn-design.css.map:1:74: Missed semicolon> 1 | {"version":3,"sources":["demo/style/index.scss","demo2/style/index.scss"],"names":[],"mappings":"AAAA;EAEI,UAAU;EACV,aAAa;EACb,mBAAmB;EACnB,2BAA2B,EAAA;;ACL/B;EAEI,UAAU;EACV,aAAa;EACb,mBAAmB;EACnB,2BAA2B,EAAA","file":"hiynn-design.css","sourcesContent":[".demo-container {\n  .demo-wrapper {\n    color: red;\n    display: flex;\n    align-items: center;\n    justify-content: flex-start;\n  }\n}\n",".demo-container {\n  .demo-wrapper {\n    color: red;\n    display: flex;\n    align-items: center;\n    justify-content: flex-start;\n  }\n}\n"]}
    |                                                                          ^
Details:
    domainEmitter: [object Object]
    domain: [object Object]
    domainThrown: false

[09:26:26] 'compile' errored after 308 ms
[09:26:26] The following tasks did not complete: copy-sass, copy-indexjs, compile-sass
[09:26:26] Did you forget to signal async completion?
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
That's a useful answer
Without any help

I changed gulp-cssnano to gulp-clean-css , It's work!

gulp.task("dist", () => {
  return gulp
    .src(sassDir)
    .pipe(sourcemaps.init())
    .pipe(sass())
    .pipe(autoprefixer())
    .pipe(concat(`${name}.css`))
    .pipe(size())
    .pipe(sourcemaps.write("."))
    .pipe(gulp.dest(distDir))

    .pipe(concat(`${name}.min.css`))
    .pipe(size())
    .pipe(cleanCSS())
    .pipe(sourcemaps.write("."))
    .pipe(gulp.dest(distDir));
});