模拟点击，调用 el-cascader 的公开方法 togglePopperVisible 来展开下拉框

MaterialOut.vue

<script setup lang="ts" name="MaterialOut">
......
import { ElMessage, type ElCascader } from "element-plus";

// 级联组件实例
const cascaderRef = ref<InstanceType<typeof ElCascader> | null>(null);

// 记账
const onJzClick = async () => {
  if (!ckMaster.value.llPersonId) {
    ElMessage.error("请选择领取人员！");
    // 模拟点击，调用 el-cascader 的公开方法来展开下拉框
    cascaderRef.value?.togglePopperVisible(true);
    return;
  }
};
......
</script>

<template>
......
              <el-cascader
                v-else
                ref="cascaderRef"
                v-model="llDeptPersonValue"
                :options="departmentUserTreeData"
                :show-all-levels="false"
                clearable
                @change="handleLlDeptPersonValueChange" />
......
</template>