해결 방법
package.json에서 proxy 설정
react app 끄기
아래 명령어 차례로 입력
# rm -r package-lock.json
# rm -r node_modules
# npm install
./src/_metronic/assets/sass/style.scss (./node_modules/css-loader/dist/cjs.js??ref--5-oneOf-6-1!./node_modules/postcss-loader/src??postcss!./node_modules/resolve-url-loader??ref--5-oneOf-6-3!./node_modules/sass-loader/dist/cjs.js??ref--5-oneOf-6-4!./src/_metronic/assets/sass/style.scss)
SassError: Undefined variable.
╷
142 │ values: $utilities-border-colors
│ ^^^^^^^^^^^^^^^^^^^^^^^^
╵
node_modules\bootstrap\scss\_utilities.scss 142:15 @import
src\_metronic\assets\sass\_init.scss 19:9 @import
src\_metronic\assets\sass\style.scss 9:9 root stylesheet
// 변경전
// Bootstrap initializaton
@import '~bootstrap/scss/functions';
@import '~bootstrap/scss/variables';
@import '~bootstrap/scss/mixins';
@import '~bootstrap/scss/utilities';
// 변경 후
// Bootstrap initializaton
@import "~bootstrap/scss/functions";
@import "~bootstrap/scss/variables";
@import "~bootstrap/scss/maps";
@import "~bootstrap/scss/mixins";
@import "~bootstrap/scss/utilities";
@import "~bootstrap/scss/helpers";
@import "~bootstrap/scss/utilities/api";