install robotstxt-webpack-plugin.js

npm i robotstxt-webpack-plugin
in your quasar.conf.js
import

 const RobotstxtPlugin = require("robotstxt-webpack-plugin");
const options = {
  filePath: "/robots.txt",
  policy: [
    {
      userAgent: "*",
      Allow: 'All',
      crawlDelay: 30
    }
  ],
  sitemap: "https://example.com/sitemap.xml",
  host: "https://example.com"
};

  build: {
      scopeHoisting: true,
      vueRouterMode: "history", // available values: 'hash', 'history'
      showProgress: true,
      gzip: false,
      analyze: false,
      // Options below are automatically set depending on the env, set them if you want to override
      // preloadChunks: false,
      extractCSS: true,

      // https://quasar.dev/quasar-cli/cli-documentation/handling-webpack
      extendWebpack(cfg) {
        cfg.plugins.push(
          new RobotstxtPlugin(options)
        );
      }
    },

npm install -g @quasar/cli

facing the same issue