应该是 I am probably not a good person.
I probably am not a good person 也可以,但没有上面那么常用.

这个句子其实不是not放在哪里,而是probably放在哪里.
not是跟在am后面的,副词probably的位置比较灵活,可以是：
Probably I am not a good person.
I am probably not a good person.
I am not a good person probably.