You Recently Started Using Categories in Your MovableType Blog...

Go to Settings -> Publishing, then scroll down to "Archive Mapping," and make sure the box next to "Category" is checked. Save your changes.

You can rebuild your site now, and the error should go away. If you'd like to add a bit of code that tells your readers what cagetory each item is filed under, do the following:

Look for the following line of code:

<pre></MTEntryIfAllowPings>

It's about 1/4 of the way down. Hit CTRL-F to search for /MTEntryIfAllowPings

After it, insert the following:

<br>(Filed under <a name="<$MTEntryID pad="1"$>"></a>
<MTEntryCategories glue=", ">
<a href="<$MTCategoryArchiveLink$>"><$MTCategoryLabel$></a>
</MTEntryCategories>)

The spacing here will probably be different from what you see, but the spacing doesn't really matter.

Once you've made the changes, that region of code should read...

</MTEntryIfAllowPings>
<br>(Filed under <a name="<$MTEntryID pad="1"$>"></a>
<MTEntryCategories glue=", ">
<a href="<$MTCategoryArchiveLink$>"><$MTCategoryLabel$></a>
</MTEntryCategories>)
</div>

</div>

</MTEntries>
</pre>
Save your changes, and rebuild as before.