Doc says i have septum/unicorn uterus and there is a 99% chance i can't give birth vaginally. (had 1 csection already) what does this mean?

Bicornuate uterus: Women with unicornate uterus cannot carry a pregnancy to term, I assume you are talking about "Bicornuate or septate uterus" since you already went through one pregnancy. Bicornuate uterus has 2 cavities and in a septate uterus, your uterine cavity is divided by a septum, these can be differentiated by imaging studies like MRI. Some women deliver vaginally, please discuss with your OBGyn.