React, getting could not proxy request error

There is an application. Installed modules and dependencies. But when I try to start the application, the API does not work - it writes a proxy error. (Proxy error: Could not proxy request).

I rummaged through a bunch of everything on the Internet, there is an article on en.stackover, but damn it, I seem to be doing everything right, but the result is the same. I can not understand.

My package.json file:

{
  "name": "app",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "@babel/core": "7.12.3",
    "@pmmmwh/react-refresh-webpack-plugin": "0.4.2",
    "@svgr/webpack": "5.4.0",
    "@testing-library/jest-dom": "^5.11.6",
    "@testing-library/react": "^11.2.2",
    "@testing-library/user-event": "^12.2.2",
    "@typescript-eslint/eslint-plugin": "^4.5.0",
    "@typescript-eslint/parser": "^4.5.0",
    "babel-eslint": "^10.1.0",
    "babel-jest": "^26.6.0",
    "babel-loader": "8.1.0",
    "babel-plugin-named-asset-import": "^0.3.7",
    "babel-preset-react-app": "^10.0.0",
    "bfj": "^7.0.2",
    "camelcase": "^6.1.0",
    "case-sensitive-paths-webpack-plugin": "2.3.0",
    "chokidar": "^3.5.2",
    "css-loader": "4.3.0",
    "datatables.net": "^1.10.23",
    "datatables.net-dt": "^1.10.23",
    "dotenv": "8.2.0",
    "dotenv-expand": "5.1.0",
    "eslint": "^7.11.0",
    "eslint-config-react-app": "^6.0.0",
    "eslint-plugin-flowtype": "^5.2.0",
    "eslint-plugin-import": "^2.22.1",
    "eslint-plugin-jest": "^24.1.0",
    "eslint-plugin-jsx-a11y": "^6.3.1",
    "eslint-plugin-react": "^7.21.5",
    "eslint-plugin-react-hooks": "^4.2.0",
    "eslint-plugin-testing-library": "^3.9.2",
    "eslint-webpack-plugin": "^2.1.0",
    "express": "^4.17.1",
    "file-loader": "6.1.1",
    "fs-extra": "^9.0.1",
    "html-webpack-plugin": "4.5.0",
    "identity-obj-proxy": "3.0.0",
    "inputmask": "^5.0.5",
    "jest": "26.6.0",
    "jest-circus": "26.6.0",
    "jest-resolve": "26.6.0",
    "jest-watch-typeahead": "0.6.1",
    "jquery": "^3.5.1",
    "lightbox2": "^2.11.3",
    "mini-css-extract-plugin": "0.11.3",
    "optimize-css-assets-webpack-plugin": "5.0.4",
    "pnp-webpack-plugin": "1.6.4",
    "postcss-flexbugs-fixes": "4.2.1",
    "postcss-loader": "3.0.0",
    "postcss-normalize": "8.0.1",
    "postcss-preset-env": "6.7.0",
    "postcss-safe-parser": "5.0.2",
    "prompts": "2.4.0",
    "react": "^17.0.2",
    "react-app-polyfill": "^2.0.0",
    "react-data-table-component": "^7.0.0-alpha-5",
    "react-dev-utils": "^11.0.1",
    "react-dom": "^17.0.2",
    "react-input-mask": "^2.0.4",
    "react-input-range": "^1.3.0",
    "react-refresh": "^0.8.3",
    "react-router-dom": "^5.2.0",
    "resolve": "1.18.1",
    "resolve-url-loader": "^3.1.2",
    "sass-loader": "8.0.2",
    "semver": "7.3.2",
    "slick-carousel": "^1.8.1",
    "style-loader": "1.3.0",
    "styled-components": "^5.3.0",
    "terser-webpack-plugin": "4.2.3",
    "ts-pnp": "1.2.0",
    "url-loader": "4.1.1",
    "webpack": "4.44.2",
    "webpack-dev-server": "3.11.0",
    "webpack-manifest-plugin": "2.2.0",
    "workbox-webpack-plugin": "5.1.4"
  },
  "scripts": {
    "start": "node --max-http-header-size=14120024 --max-old-space-size=20096 scripts/start.js",
    "build": "node scripts/build.js",
    "test": "node scripts/test.js"
  },
  "eslintConfig": {
    "extends": [
      "react-app",
      "react-app/jest"
    ]
  },
  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  },
  "proxy": "http://localhost:3000/",
  "jest": {
    "roots": [
      "<rootDir>/src"
    ],
    "collectCoverageFrom": [
      "src/**/*.{js,jsx,ts,tsx}",
      "!src/**/*.d.ts"
    ],
    "setupFiles": [
      "react-app-polyfill/jsdom"
    ],
    "setupFilesAfterEnv": [],
    "testMatch": [
      "<rootDir>/src/**/__tests__/**/*.{js,jsx,ts,tsx}",
      "<rootDir>/src/**/*.{spec,test}.{js,jsx,ts,tsx}"
    ],
    "testEnvironment": "jsdom",
    "testRunner": "/Users/user/Downloads/goroskop2/node_modules/jest-circus/runner.js",
    "transform": {
      "^.+\\.(js|jsx|mjs|cjs|ts|tsx)$": "<rootDir>/node_modules/babel-jest",
      "^.+\\.css$": "<rootDir>/config/jest/cssTransform.js",
      "^(?!.*\\.(js|jsx|mjs|cjs|ts|tsx|css|json)$)": "<rootDir>/config/jest/fileTransform.js"
    },
    "transformIgnorePatterns": [
      "[/\\\\]node_modules[/\\\\].+\\.(js|jsx|mjs|cjs|ts|tsx)$",
      "^.+\\.module\\.(css|sass|scss)$"
    ],
    "modulePaths": [],
    "moduleNameMapper": {
      "^react-native$": "react-native-web",
      "^.+\\.module\\.(css|sass|scss)$": "identity-obj-proxy"
    },
    "moduleFileExtensions": [
      "web.js",
      "js",
      "web.ts",
      "ts",
      "web.tsx",
      "tsx",
      "json",
      "web.jsx",
      "jsx",
      "node"
    ],
    "watchPlugins": [
      "jest-watch-typeahead/filename",
      "jest-watch-typeahead/testname"
    ],
    "resetMocks": true
  },
  "babel": {
    "presets": [
      "react-app"
    ]
  }
}

Project structure:

-api
-config 
-node_modules 
-public
-scripts 
-src 
-upload
-vendor
package.json 
package-lock.json

If someone could suggest I would be extremely grateful. What I tried so far:

Put in proxy
http: // localhost
http: // localhost /
http: // localhost: 3000
http: // localhost: 3000 /

Run serve -s build.

In this case, it runs on port 3000, and when the application starts, when accessed via the API, it issues an html page (from the build directory)

I also tried to run node scripts / start.js and npm start separately.
As planned - it came out nonsense, but I've already tried everything.