"Notice: Undefined index: und in include()"
The line that is causing that error message is the following one.

&lt;?php if (($type == "article") &amp;&amp; ($node-&gt;field_add_taxonomy_tag_articles[$node-&gt;language][0]['tid'] == 26)): ?&gt;


'und' is the value for the constant LANGUAGE_NONE, which is used for $node-&gt;language, when the node is language-neutral.  

When you want to obtain the value of a field, you should use field_get_items(), as in the following code.

&lt;?php $items = field_get_items('node', $node, 'field_add_taxonomy_tag_articles'); ?&gt;
&lt;?php if (($type == "article") &amp;&amp; ($items[0]['tid'] == 26)): ?&gt;