C'est/ elle

Séamud, 

although this is in the negative, it is the same 'rule' as in the positive sense. 

You will almost never use il/elle est (determiner/article) noun - so in this example, 'elle n'est pas UNE VOITURE ..' would be incorrect, and ' ce n'est pas UNE VOITURE .... ' is correct. 

See the lesson linked below from this site, and I also recommend reading the discussion by Camille on French Today link - it helps to have different approaches sometimes to reinforce the learning.

 

 "C'est" vs "Il/Elle est" to say it is/she is/he is in French 

 https://www.frenchtoday.com/blog/french-grammar/cest-versus-il-elle-est/