DATA LIN LIKE LINE OF bsad_tb.
data kna1_str like KNA1.

LOOP AT bsad_tb INTO LIN.
CLEAR kna1_str.
   SELECT  SINGLE *
     INTO  kna1_str
     FROM  KNA1
    WHERE  KUNNR  = LIN-KUNNR.

     read table KNA1_TB WITH KEY KUNNR = LIN-KUNNR.
     IF SY-SUBRC <> 0.
      MODIFY KNA1_TB FROM kna1_str INDEX SY-TABIX .
     ENDIF.
ENDLOOP.

Error in ABAP statement when processing an internal table.

-------------
Анализ ошибки


When changing or deleting one or more lines of the internal table
"\PROGRAM=ZFI_BSAD_AND_BSID=============FT\DATA=%_OKNA1_TB[]" or when inserting
in the table "\PROGRAM=ZFI_BSAD_AND_BSID=============FT\DATA=%_OKNA1_TB[]", 0
was used as
the line index. An index less than or equal to zero is not
allowed.

The error can occur when using the following options:
1. "INDEX idx" for specifying the line number in the table
"\PROGRAM=ZFI_BSAD_AND_BSID=============FT\DATA=%_OKNA1_TB[]"
   where you want to change, insert or delete.
2. "FROM idx" for specifying the start index when deleting a line
   area from or inserting a line area into the table
"\PROGRAM=ZFI_BSAD_AND_BSID=============FT\DATA=%_OKNA1_TB[]".
3. "TO idx" for specifying the end index when deleting a line
   area from or inserting a line area into the table
"\PROGRAM=ZFI_BSAD_AND_BSID=============FT\DATA=%_OKNA1_TB[]".

When the program terminated, the table had 0 lines.
-----------------------------------------------------------
000310   LOOP AT bsad_tb INTO LIN.
000320   CLEAR kna1_str.
000330      SELECT  SINGLE *
000340        INTO  kna1_str
000350        FROM  KNA1
000360       WHERE  KUNNR  = LIN-KUNNR.
000370
000380        read table KNA1_TB WITH KEY KUNNR = LIN-KUNNR.
000390        IF SY-SUBRC <> 0.
000400          MODIFY KNA1_TB FROM kna1_str INDEX SY-TABIX .
     >             INSERT KNA1_str INTO KNA1_TB INDEX SY-TABIX.
000420
000430        ENDIF.
000440   ENDLOOP.
000450