having problems with a simple query

This returns an Attribute 'acquisition' not found error


SELECT trx_date,
       store_id || rgstr_id || trx_num AS invoice,
       substring(po_num from 1 for (position('/' in po_num) - 1)) AS po,
       substring(po_num from (position('/' in po_num) + 1) for (char_length(po_num) - position('/' in po_num))) AS
acquisition
       FROM SS_AHST WHERE cus_id = '410BGEHD47' AND acquisition LIKE '%A1158%' ORDER BY trx_date DESC


does anyone know what I'm doing wrong

Thanks
Tad