Hi,
Change the line:
$xml.="\t".'<g:mpn><![CDATA[ '.str_replace(array(' ','-'),array('',''),$product->product_code).' ]]></g:mpn>'."\n";
to:
$xml.="\t".'<g:mpn>'.str_replace(array(' ','-'),array('',''),$product->product_code).'</g:mpn>'."\n";
in the file plugins/hikashop/google_products.php and that should avoid the problem.
However, I don't think that your MPN/product code is valid.
Beware that even though it should work with that code change, if the MPN value is not correct it is ground for removal of your products from the listing of Google.
support.google.com/merchants/answer/188494?hl=en