Is there an informal way to describe a woman that can not have a baby?

I wouldn't use a particular word, I'd just say, "She can't have a baby."
I would never say something like, "She is infertile/fruitless/barren/sterile." – I find that extremely rude.

Historically, a woman with such a condition was considered to be barren, but that word is not used much today (although it is nevertheless likely to be understood). Nowadays, one would probably say she is "seeing a fertility counselor" or some such circumlocution, and people would infer the rest.

I have always used the word barren, although that word is a bit antiquated.

For some, the word barren may carry a negative connotation. For instance, when the word is used in the Bible it's written as if it was a curse. There are Biblical stories of women who were barren but then were blessed by God and were able to conceive. Consequently, someone who is Jewish/Christian/Muslim might find that particular word a bit insensitive, as it seemingly implies they are cursed.

Perhaps the best way to avoid any potential embarrassment is to state the facts plainly, as in, "She is unable to conceive." Even though most people will understand your intent, I'm not a fan of saying, "She cannot have a baby," because that can be taken literally to mean that she is not fit to mother. Just because someone cannot conceive does not mean they cannot have a child through alternative means, such as adoption or surrogacy.

Joke answers: unbearable, impregnable, inconceivable (maybe that doesn't mean what I think it means...)