Hi,
I have just replicated your issue as below by without passing quantity to schedule lines in BAPI.
To correct the issue add the below code also in your program which is there in my previous code.
* Schedule lines
lv_etenr = lv_etenr + 10.
wa_sales_schedules-itm_number = lv_kposn.
wa_sales_schedules-sched_line = lv_etenr.
wa_sales_schedules-req_date = sy-datum.
wa_sales_schedules-req_qty = gs_tcmatnr-quan.
append wa_sales_schedules to it_sales_schedules.
clear wa_sales_schedules.
wa_sales_schedulesx-itm_number = lv_kposn.
wa_sales_schedulesx-sched_line = lv_etenr.
wa_sales_schedulesx-req_date = c_x.
wa_sales_schedulesx-req_qty = c_x.
append wa_sales_schedulesx to it_sales_schedulesx.
clear wa_sales_schedulesx.