In the code for Group by, there are 5 columns including aggregate SUM, but the group by statement has only four columns which is incorrect. it should be 1 less than actual.
Try the below code.
And do share the error that you are receiving.
SELECT Invoice.iid, Invoice.idate, customer.cname, rep.sfname,SUM( invoice_stock.qty* invoice_stock.selling)
FROM rep INNER JOIN ((customer INNER JOIN Invoice ON customer.cid = Invoice.cid) INNER JOIN invoice_stock ON Invoice.iid = invoice_stock.iid) ON rep.repid = customer.repid
GROUP BY Invoice.iid, Invoice.idate, customer.cname, rep.sfname;