Hi Gyanendra,
The SPRO path you have shared is for account balance (FPL9) screen layout.
Please follow below SPRO paths for document create/change line layout variants and fields.
SPRO -> Financial Accounting (New) -> Contract Account Receivable and Payable -> Basic Functions -> Postings and Documents -> Document -> Screen Preparations ->
Define Screen Variant for List Entry of Business Partner Items
And,
Define Screen Variant for List Entry of General Ledger Items
You should have entries maintained there and those variants should be visible in the F4 help against the corresponding Line Layouts in FPE1/FPE2 screens.
Hope it helps.
Thanks,
Bodhisattwa