Neither of those sentence is correct. A correct sentence would be

Je n’ai pas de voiture

Which breaks down into

• Je → I
• n’ → a contraction of ne, first part of the negative cluster ne pas
• ai → avoir first person singular, présent simple de l’indicatif which translates to present have
• pas → second part of the negative cluster ne pas
• de voiture → a car. I won’t go into subtleties such as why it is de and not une, as it is off-topic here

The first issue with your sentence Je is replaced by J’ai which is a (mandatory) contraction for *Je ai. It could be a mishearing, but it is fundamentally wrong, as it introduce an extraneous verb in the sentence, as if you wrote in English “I have do not have a car”.

In addition to this, the a is a conjugation mistake. The first person singular présent simple de l’indicatif of avoir is ai, not a. As is, it is as if you wrote “I don’t has a car”. For future reference, as avoir is one of the most irregular verbs in French, if in doubt, check a conjugation table.