This will get the value of an input attribute or textarea. If you have a dropdown you should use the text type, so add this line in the list of arguments:
<referenceBlock name="product.info.main">
<block class="Magento\Catalog\Block\Product\View\Description" name="product.info.brand" template="product/view/attribute.phtml" before="-">
<arguments>
<argument name="at_call" xsi:type="string">getBrand</argument>
<argument name="at_code" xsi:type="string">brand</argument>
<argument name="css_class" xsi:type="string">brand</argument>
<argument name="at_label" xsi:type="string">none</argument>
<argument name="add_attribute" xsi:type="string">itemprop="brand"</argument>
</arguments>
</block>
</referenceBlock>