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