Hello,
this problem was discussed multiple times in the forum. Please use the search first before posting a new support thread.
Quick summary: there is no option for it and it’s not possible within InvoicePlane by default. You have to enter discounts as separate items.
https://community2.invoiceplane.com/t/topic/3491
https://community2.invoiceplane.com/t/wrong-calculation-order/1973
https://community2.invoiceplane.com/t/calculate-invoice-discount-before-invoice-tax/1245