Your account may not be eligible to advertise on Twitter because it must first meet Twitter’s eligibility criteria. For example, Twitter limits advertisers to certain countries and territories. The reason for this is due to US export control laws and trade sanctions. You can see a list of these at business.twitter.com.
Your account must also be a public account (not private) and it cannot be deactivated or suspended. I would recommend you review “Twitter’s Ads Policies Page” for more information. And, if you still cannot understand why your account is not eligible to advertise on Twitter, you can open a support ticket at ads.twitter.com.