Can I pronounce the /ɑ/ sound instead of the /ɔ/ in all words which have the /ɔ/ sound?
Because I can't pronounce the /ɔ/ and I heard even some Americans don't make this sound and they make /ɑ/ sound instead.
Actually about half of AmE speakers don't use /ɔ/. It is called the COT-CAUGHT merger: for those speakers 'caught' has /ɑ/ just like 'cot'. So there is no problem using that pronunciation: it is very common.
In some American accents you are right, there are no distinction, and they use the same sound.
Anyway I recommend you not to get discouraged, and keep trying to pronounce the /ɔ/ sound. As nonnative speakers we have to try to speak as clear as possible and those accents sometimes confuse even native speakers with some words.
To master the [ɔ] sound try this video. It will take you days or weeks but you will do it. If you can do the /ɑ/ sound you are halfway through.