Hi,
1. That's because some of the products with a price <= 10 are already linked to that category.
So please add a filter so that the products aren't already in that category and you shouldn't get that error.
2. For that error, it's because the system doesn't support having two filters on the price.
Try changing the line:
$query->leftjoin[] = hikashop_table('price').' AS hk_price ON hk_price.price_product_id = hk_product.product_id';
to:
$query->leftjoin['hk_price'] = hikashop_table('price').' AS hk_price ON hk_price.price_product_id = hk_product.product_id';
in the file plugins/hikashop/massaction_product/massaction_product.php and that should fix the problem.
Let us know how it goes so that we can include the fix for the next release.