Insufficient Content & Duplication of Content Errors.
If a site is found to have insufficient content, this means that the site may not have enough content, and/or the site was deemed to be “under construction.” To be approved for AdSense and show relevant ads on your site, your pages need to have enough content on them for AdSense specialists to review and for our crawler to be able to determine what your pages is all about?

Sites that contain mostly Duplicated or Copied images, it’s not the case you can’t copy and paste content you need to rewrite those contents , videos or Flash animations may not be approved or have their ads stopped.

Additionally, sites that consist bad SEO friendly template, improper Navigation menu and very little content may not be approved or have their ads stop displaying.

How to Fix it Insufficient Content & Duplication of Content Errors.
In other to avoid fewer indexed pages. That may encounter in your webmaster tool, google indexing few of the pages (post, page etc.) of your website.

It highly recommended that you always publish SEO friendly articles either Original content or rewrite articles above 500 words on your website. Because Google do penalized duplicate of contents.

Already Exist Google AdSense Account Issues
Another most common error most publishers faced is the account duplicate warning from Google AdSense which can lead to the banning of ads account.

If you apply for AdSense and you receive a reply AdSense Account already Exist Error, you don’t need to panic about the error you can actually fix it.

Read More on
https://www.zenithtechs.com/adsense-violations-issues-to-avoid-in-2019/