try add to your quasar.config

css: [
  'app.scss',
  '~quasar-ui-qcalendar/src/css/calendar-day.scss' // <---ADD
],
//-------
build: {
  webpackTranspile: true,
  webpackTranspileDependencies: [ /quasar-ui-qcalendar[\\/]src/ ]  // <---ADD
}

More information: https://qcalendar.netlify.app/getting-started/installation