The Sales ledger field will not appear below the Party A/c name field when:
The transaction is recorded with voucher class in the accounting invoice mode.
The option Select common Ledger Account for Item Allocation is disabled in sales Voucher Configuration screen.
To view the Sales ledger field
In the sales voucher, press F12 (Configure) > set the option Select common Ledger Account for Item Allocation to Yes.
Press Ctrl+A to save the screen. Now, the Sales ledger field will appear during voucher entry.