The problem I have had with learning Japanese is that for many things people say different things and a lot will tell you that there way is the right way and what you should say. It can be very confusing when you first start learning.
When I first came to Japan, I was learning things like counting. When it came to counting floors 一階 二階 三階 四階 I was told by my GF that it
must be ikkai, nikai, san
gai yonkai etc. When asked about why it must be gai for 3 instead of kai she said that sankai would be too difficult to say (what about 三回 sankai, 3 times I said to no response). Subsequent teachers told me only sankai, only sangai, both are okay & I was confused to put it mildly (read politely) Typing on a computer if I write sankai or sangai they both come up with the kanji 三階 so I assume both are used. My point is so many people say different things, just like in any language. I know native japanese speakers who cannot understand anything that high school students say. I know high school students who can't understand what the next group of high school students are saying
It's a difficult language often made more difficult by teachers and textbooks.
I know a lot of people who studied Japanese at school in Australia or America, then came to Japan and nobody has a clue what they are saying. Too textbook you might say
My advice is to first work on how to communicate and not worry about sounding natural, then when you have reached a certain level, worry about sounding natural.
じゃありません though I have never heard and my PC doesn't want to convert it either. Still, I bet some kids have started saying it and it might be the next big thing.