"I am sorry, I did not know."
Translation:Вибачте, я не знав.
To use feminine - surely possible. To say "вибачаюсь" - well, that's not a preferable way to say that you are sorry. Ending "сь" makes the verb reflexive, so it's like you are trying to forgive youself. This form does exist colloquially, but I would advise against using it. You've just started learning the language, so why learn something that is not really right :)
Thank you both! It is just amazing how fast one gets replies to questions in this course. And with an explanation :) I just can't stop studying... I was already thinking that it's time to stop for this evening but since I keep getting replies, I will continue for a few more lessons :D