I have already done it. I'm sure it's unlocked. yes, that padlock pic is open.

When it's bricked I tried to boot it but it only showed "Google" logo and small lock picture under and it just stay frozen like that.

ok, I'll try what you said. I used it on linux mint 19.3 and windows 7, and I used different cables. The versions I used are 0.4.14 and 0.4.16. I just found someone used ubuntu-device-flash to install. I will try that later.
In an attempt I bricked it, so I used nexus root toolkit to unbrick it with jelly bean(it came with marshmallow when I received it.)

@Lakotaubp It still doesn't work.

Hi! I used ubport installer and all the steps before are normal. The installer detect my nexus 7 automatically and made it boot into bootloader, but then it just keep showing "flashing firmware" and then nothing happened. How do I solve thin? Thanks!