org 100h

        call open_new_file
        mov ah, 40h;write
        mov bx, [new_file_handle]
        mov cx,2;write 2 bytes
        mov dx, temp
        int 21h

        mov ah, 3eh;close file
        mov bx, [new_file_handle]
        int 21h

        mov ah, 00
        int 16h;wait key

        mov ax, 4c00h
        int 21h;exit

open_new_file:
        push bp
        mov bp, sp
        mov ah, 3dh;open file
        mov al, 02h;write only
        mov dx, new_file_name ;out.kty
        int 21h
        mov [new_file_handle], ax
        leave
        ret


temp:
        dw 0xffaa

new_file_handle:
        dw 0
new_file_name:
        db "out.bin", 0
    

INT 21h / AH= 3Dh opens an existing file so you have to create it first.

Picnic wrote:

INT 21h / AH= 3Dh opens an existing file so you have to create it first.

I got it thanks!

--------D-213C-------------------------------
INT 21 - DOS 2+ - "CREAT" - CREATE OR TRUNCATE FILE
        AH = 3Ch
        CX = file attributes (see #01401)
        DS:DX -> ASCIZ filename
Return: CF clear if successful
            AX = file handle
        CF set on error
            AX = error code (03h,04h,05h) (see #01680 at AH=59h/BX=0000h)
Notes:  if a file with the given name exists, it is truncated to zero length
        under the FlashTek X-32 DOS extender, the pointer is in DS:EDX
        DR DOS checks the system password or explicitly supplied password at
          the end of the filename against the reserved field in the directory
          entry before allowing access
SeeAlso: AH=16h,AH=3Dh,AH=5Ah,AH=5Bh,AH=93h,INT 2F/AX=1117h

Bitfields for file attributes:
Bit(s)  Description     (Table 01401)
 0      read-only
 1      hidden
 2      system
 3      volume label (ignored)
 4      reserved, must be zero (directory)
 5      archive bit
 7      if set, file is shareable under Novell NetWare