In item-post.php find this line :
<div class="row">
<label for="countryId"><?php _e('Country', 'modern'); ?></label>
<?php ItemForm::country_select(osc_get_countries(), osc_user()); ?>
</div>
and replace with this
<input type="hidden" name="countryId" value="RO" />
Value RO is the country Id, change with your country. ex US, IT,