Makefile:7: *** missing separator. Stop

Question

I wrote a script and it said thatMakefile:7: *** missing separator. Stop.

ASM=nasm

SRC_DIR=src
BUILD_DIR=build

$(BUILD_DIR)/main_floppy.img: $(BUILD_DIR)/main.bin
    cp $(BUILD_DIR)/main.bin $(BUILD_DIR)/main_floppy.img
    truncate -s 1440k $(BUILD_DIR)/main_floppy.img
    
$(BUILD_DIR)/main.bin: $(SRC_DIR)/main.asm
    $(ASM) $(SRC_DIR)/main.asm -f bin -o $(BUILD_DIR)/main.bin

I tried to write this script like a 'Makefile'. I wrote that ´$ make´. I expected to get a '.img'-file.

It should assembel that code:

org 0x7C00
bits 16


main:
    hlt

.halt:
    jmp .halt

times 510-($-$$) db 0
dw 00AA55h