1054 - Unknown column 'os.downloads_flag' in 'where clause'

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what is this error and how do i fix it

 

select date_format(o.date_purchased, '%Y-%m-%d') as date_purchased_day, opd.download_maxdays, op.products_name, opd.orders_products_download_id, opd.orders_products_filename, opd.download_count, opd.download_maxdays from orders o, orders_products op, orders_products_download opd, orders_status os where o.customers_id = '26' and o.orders_id = '248' and o.orders_id = op.orders_id and op.orders_products_id = opd.orders_products_id and opd.orders_products_filename != '' and o.orders_status = os.orders_status_id and os.downloads_flag = '1' and os.language_id = '1'