"I do not have a girlfriend."
The verb to have makes an exception. It's negation is made with 没 instead of 不。It's 不是 and 没有。
2019-01-19: Said “我没有一个女朋友” and it was not accepted.
Please correct me if I am wrong, but I believe you should be able to include "一个" as it translates to "one" or "a." Or would this only be used to indicate, "I don't have one girlfriend," implying you have multiple?
for some reason, i'm not able to find the character for nu3 when typing it with my keyboard. Is 我没有奴旁友 correct ? (I can't write 女 when typing 'nu')