Looking into this – by default, GeneratePress adds to the necessary elements into the footer.
When using the Copyright add-on to replace the copyright content, these elements are removed in favor of your custom elements.
For example, in the default copyright message, we have this line:
<a itemprop="url" title="GeneratePress" target="_blank" href="http://generatepress.com">GeneratePress</a>
The itemprop=”url” part makes it valid microdata.
That being said – this warning won’t affect your SEO results at all, it’s just a suggestion by Google π