Just try to run your sub queries one by one individually and check which one is returning more than one value and why.there must be something wrong in the condition or duplicate data:
((Select IFNULL(sum(QtyAvailable), 0) from SparesQty where SparesQty.Code = spare_list.Number)
(select indentitems.qty from indentitems left join indent on indentitems.code=indent.itemcode where indentitems.indent=16 and indent.status=1))