The problem is that you use IN operator which, in this case, expects one set of values to work on, but the subquery returns two columns, not one:
WHERE asset.prom_integ_id IN
--Traversing Down to find the other pair
( SELECT asset.prom_integ_id, ROW_NUMBER() OVER (PARTITION BY cb.ssid ORDER BY cb.callback_date DESC)
Should be:
WHERE asset.prom_integ_id IN
--Traversing Down to find the other pair
( SELECT asset.prom_integ_id
And, starting with that, I guess that the value returned from ROW_NUMBER should be accessed earlier, so it would look like this:
SELECT cb.ssid, cb.created_by, cb.callback_date, cb.callback_num, asset.desc_text, asset.prom_integ_id, asset.integration_id
--Traversing Up to find the Grandparent record of
FROM SIEBEL.CX_ASSET_XM cb
JOIN SIEBEL.S_ASSET_X assetx
ON cb.SSID = assetx.attrib_37
JOIN SIEBEL.S_ASSET asset
ON assetx.par_row_id = asset.row_id
WHERE asset.prom_integ_id IN
--Traversing Down to find the other pair
( SELECT prom_integ_id
FROM (
SELECT asset.prom_integ_id, ROW_NUMBER() OVER (PARTITION BY cb.ssid ORDER BY cb.callback_date DESC) row_n
FROM SIEBEL.CX_ASSET_XM cb
JOIN SIEBEL.S_ASSET_X assetx
ON cb.ssid = assetx.attrib_37
JOIN SIEBEL.S_ASSET asset
ON assetx.par_row_id = asset.row_id
) where row_n = 1
);
I'm not sure it is what you wanted - can you check it?