When you can add detail lines depends on how your Invoice Register is configured. Transactions may need to be approved before lines can be added. Check the Invoice Register options in the settings module.
Add a detail line when you raise the transaction
Click Creditors, then expand the Invoice Register menu.
Click Purchase Ledger Entry and select the supplier code from the lookup.
Click Enter Details Now in the Header area just below the Currency box.
The Invoice Register Transaction Detail area becomes visible.
Add a detail line after it's been raised
Click Creditors, then click Invoice Register.
Click Update Invoices.
Find the invoice, then click the pop-over icon next to the audit number.
Click Enter Detail. This opens a new window.
Enter the detail lines. These need to match the header details. The summary provides the calculations.
Once done, click Save.