Vue is not defined Bootstrap-vue
If you wanted to use Vue.use() you needed to import Vue from 'vue'
Or import { createApp, use } from 'vue' and use use() function instead of Vue.use()
And you might needed to reconsider on import orders.
Eg:
import { createApp, use } from 'vue'import App from './App.vue'import router from './router'import BootstrapVue from 'bootstrap-vue'import 'bootstrap/dist/css/bootstrap.css'import 'bootstrap-vue/dist/bootstrap-vue.css'use(BootstrapVue)createApp(App).use(router).mount('#app')Or you could import whole vue shown below
import Vue from 'vue'import App from './App.vue'import router from './router'import BootstrapVue from 'bootstrap-vue'import 'bootstrap/dist/css/bootstrap.css'import 'bootstrap-vue/dist/bootstrap-vue.css'Vue.use(BootstrapVue)Vue.createApp(App).use(router).mount('#app')