[nuxt] A composable that requires access to the Nuxt instance was called outside of a plugin, Nuxt hook, Nuxt middleware, or Vue setup function. This is probably not a Nuxt bug. Find out more at https://nuxt.com/docs/guide/concepts/auto-imports#vue-and-nuxt-composables.

// Вот тут setup забыл
<script setup lang="ts">
import { ref } from 'vue';

const active = ref(false);

function setActive(val: boolean) {
    active.value = val
}
</script>

<li
class="header-nav-item"
@mouseover="setActive(true)"
@mouseout="setActive(false)">
<a href="/" class="header-nav-link">Каталог</a>
<div class="header-nav-dropmenu" :class="{ active }">
<a href="">Подкатегория</a>
<a href="">Подкатегория</a>
<a href="">Подкатегория</a>
<a href="">Подкатегория</a>
</div>
</li>