Element tbody not allowed as child of element table in this context.

This error appears if you came across the invalid advice to put tfoot before tbody.

I do not know where it comes from - outdated information or a bad translation - but the fix is simple: move tfoot where it belongs, namely

<table>
<caption>Table caption</caption>
<thead>
<tr><th></th></tr>
</thead>
<tbody>
<tr><td></td></tr>
</tbody>
<tfoot>
<tr><td></td></tr>
</tfoot>
</table>