Here is more info about the use of "bukan" and "tidak" :

https://forum.duolingo.com/comment/31291121

What is 'Saya adalah tidak seorang guru' a wrong answer?

Why is human correct and tidal not? I fail to grasp the difference between the 2

Not sure why in the previous question teacher is defined as a teacher of people (seorang guru) but in this question guru by itself is fine.