You're unable to add discounts, credit notes, or negative amounts in general.
First, prepare and ensure everything looks good in your spreadsheet. Here's how:
Fill in the required columns: Invoice number, Customer, Invoice date, Due date, Item amount, and Item tax code. If your invoices have multiple line items, ensure each line entry is similar in the sample spreadsheet.
If you collect VAT, enter the VAT rate shown on your invoices into the spreadsheet. You can map the VAT codes you've created into QuickBooks VAT codes when importing.