Which BAPI to upload data from a flat file to VA01?

Use the BAPI's
BAPI_SALESORDER_CREATEFROMDAT1
BAPI_SALESORDER_CREATEFROMDAT2

This will create the Sales order .
*---------------------------------------------------------------------
Include YCL_CREATE_SALES_DOCU * 
*---------------------------------------------------------------------

Form salesdocu 
*---------------------------------------------------------------------
*This Subroutine is used to create Sales Order 
*----------------------------------------------------------------------

*-->P_HEADER Document Header Data 
*-->P_HEADERX Checkbox for Header Data 
*-->P_ITEM Item Data 
*-->P_ITEMX Item Data Checkboxes 
*-->P_LT_SCHEDULES_IN Schedule Line Data 
*-->P_LT_SCHEDULES_INX Checkbox Schedule Line Data 
*-->P_PARTNER text Document Partner 
*<--P_w_vbeln text Sales Document Number 

*---------------------------------------------------------------------
DATA:
lfs_return like line of t_return.
 

FORM create_sales_document changing P_HEADER like fs_header
P_HEADERX like fs_headerx
Pt_ITEM like t_item[]
t_ITEMX like t_itemx[]
P_LT_SCHEDULES_IN like t_schedules_in[]
P_LT_SCHEDULES_INX like t_schedules_inx[]
Pt_PARTNER like t_partner[]
P_w_vbeln like w_vbeln.

*This Perform is used to fill required data for Sales order creation 
perform sales_fill_data changing p_header

p_headerx
pt_item
pt_itemx
p_lt_schedules_in
p_lt_schedules_inx
pt_partner.

*Function Module to Create Sales and Distribution Document 

perform sales_order_creation using p_header
p_headerx
pt_item
pt_itemx
p_lt_schedules_in
p_lt_schedules_inx
pt_partner.

perform return_check using p_w_vbeln .

ENDFORM. " salesdocu

*---------------------------------------------------------------------
*Form commit_work 
*---------------------------------------------------------------------
*To execute external commit * 
*---------------------------------------------------------------------

FORM commit_work .
CALL FUNCTION 'BAPI_TRANSACTION_COMMIT'
EXPORTING
WAIT = c_x.

ENDFORM. " Commit_work

Include ycl_sales_order_header " To Fill Header data and Item data 
Include ycl_sales_order_header.

*---------------------------------------------------------------------
Form return_check 
*---------------------------------------------------------------------
To validate the sales order creation 
*---------------------------------------------------------------------

FORM return_check using pr_vbeln type vbeln.
if pr_vbeln is initial.
LOOP AT t_return into lfs_return .
WRITE / lfs_return-message.
clear lfs_return.

ENDLOOP. " Loop at return
else.
perform commit_work. " External Commit
 

Refresh t_return.

fs_disp-text = text-003.
fs_disp-number = pr_vbeln.
append fs_disp to it_disp.
 

if p_del eq c_x or p_torder eq c_x or
p_pgi eq c_x or p_bill eq c_x.
perform delivery_creation. " Delivery order creation

endif. " If p_del eq 'X'......

endif. " If p_w_vbeln is initial

 ENDFORM. " Return_check

*&---------------------------------------------------------------------
*& Form sales_order_creation
*&---------------------------------------------------------------------
*text 
*----------------------------------------------------------------------

*-->P_P_HEADER text 
*-->P_P_HEADERX text 
*-->P_PT_ITEM text 
*-->P_PT_ITEMX text 
*-->P_P_LT_SCHEDULES_IN text 
*-->P_P_LT_SCHEDULES_INX text 
*-->P_PT_PARTNER text 
*----------------------------------------------------------------------

FORM sales_order_creation USING P_P_HEADER like fs_header
P_P_HEADERX like fs_headerx
P_PT_ITEM like t_item[]
P_PT_ITEMX like t_itemx[]
P_P_LT_SCHEDULES_IN like t_schedules_in[]
P_P_LT_SCHEDULES_INX like t_schedules_inx[]
P_PT_PARTNER like t_partner[].

CALL FUNCTION 'BAPI_SALESDOCU_CREATEFROMDATA1'
EXPORTING

sales_header_in = p_p_header
sales_header_inx = p_p_headerx

IMPORTING
salesdocument_ex = w_vbeln

TABLES
return = t_return
sales_items_in = p_pt_item
sales_items_inx = p_pt_itemx
sales_schedules_in = p_p_lt_schedules_in
sales_schedules_inx = p_p_lt_schedules_inx
sales_partners = p_pt_partner.

ENDFORM. " sales_order_creation