If I can find sites with low da dr, fewer articles, less RD , but 0 organic traffic those websites?? Will be worthon ahref… should I compete with it??
Please Reply In Detail 
Find the top keywords of that site, and search that keywords on Google, if your competitor is not ranking on any of the keywords, ignore it. (Since Ahref sometimes doesn’t provide real time data).
Additionally, find competitors as many as possible in your niche and see which one is ranking. Once you find one, start targeting.
If ahref didn’t provide the actual data sometime so from where do I see the traffic ?? Suppose I find A niche and In that niche I find A competator… Now, I find his top organic keywords… Now, if there is no organic keywords ranking of the competator, should I ignore the Competator or the niche… Please explain in detail…
Yes you can ignore this niche, provided you didn’t find competitors that are getting traffic and you have also manually checked top keywords on Google. (Due to March update, it’s obvious to see traffic decline)