//app.js
import "./app.scss";

//app.scss
$vw_fontsize: 75;
@function rem($px) {
@return ($px / $vw_fontsize ) * 1rem;
}
$vw_design: 750;
html {
  font-size:($vw_fontsize / ($vw_design / 2)) * 100vw;
  @media screen and (max-width: 320px) {
  font-size: 64px;
  }
  @media screen and (min-width: 540px) {
font-size: 108px;
  }
}

body {
 max-width: 540px;
 min-width: 320px;
}
.mod_nav {
  background-color: -sharpfff;
  &_list {
    display: flex;
    padding: vw(15) vw(10) vw(10); // 
    &_item {
      flex: 1;
      text-align: center;
      font-size: vw(10); // 
      &_logo {
 display: block;
 margin: 0 auto;
 width: vw(40); // 
 height: vw(40); // 
 img {
   display: block;
   margin: 0 auto;
   max-width: 100%;
 }
      }
      &_name {
 margin-top: vw(2);
      }
    }
  }
}

                       ^
  Property "" must be followed by a ":"
  in C:\Users\Administrator\Desktop\my-app\src\app.scss (line 7, column 55)

Why did you report this mistake? Isn"t it said in the document that it is insensitive to spaces?
add

//webpack.config.dev
 {
              test:/\.scss$/,
                use: [
                    {
                    loader: "style-loader" //  JS  style 
                }, {
                    loader: "css-loader" //  CSS  CommonJS 
                }, {
                    loader: "sass-loader" //  Sass  CSS
                }, {
                    loader: "scss-loader" //  Sass  CSS
                    }
                ]


          }