Error compiling xv6 in Ubuntu(Makefile:124: recipe for target 'kernel' failed)

I tried to compile xv6 in my Ubuntu 18.04.3 using "make qemu-nox"

It failed with the following error. Can you give me some information about this error?

ld -m    elf_i386 -T kernel.ld -o kernel entry.o bio.o console.o exec.o file.o fs.o ide.o ioapic.o kalloc.o kbd.o lapic.o log.o main.o mp.o picirq.o pipe.o proc.o sleeplock.o spinlock.o string.o swtch.o syscall.o sysfile.o sysproc.o trapasm.o trap.o uart.o vectors.o vm.o  -b binary initcode entryothertrap.o: In function `tvinit':/home/lee/OS-Homework/trap.c:23: undefined reference to `vectors'/home/lee/OS-Homework/trap.c:24: undefined reference to `vectors'Makefile:124: recipe for target 'kernel' failedmake: *** [kernel] Error 1