Lo vedi perché ho risolto. Ho creato il file searchform.php (che non c'era) e gli ho messo dentro questo:
Codice PHP:
<form method="get" id="searchform" class="searchform" action="<?php echo esc_url( home_url( '/' ) ); ?>" role="search">
<label for="s" class="screen-reader-text"><?php _ex( 'Search', 'assistive text', 'semicolon' ); ?></label>
<input type="search" class="searchfield" name="s" value="<?php echo esc_attr( get_search_query() ); ?>" id="s" placeholder="<?php echo esc_attr_x( 'Search …', 'placeholder', 'semicolon' ); ?>" />
<input type="submit" class="submit" id="searchsubmit" value="<?php echo esc_attr_x( 'Cerca', 'submit button', 'semicolon' ); ?>" />
</form>