Unless you have the css yourself, be sure to ask it to be included
InertiaProgress.init({
// The delay after which the progress bar will
// appear during navigation, in milliseconds.
delay: 250,
// The color of the progress bar.
color: '#29d',
// Whether to include the default NProgress styles.
includeCSS: true, //this
// Whether the NProgress spinner will be shown.
showSpinner: false,
})