There are many common SEO issues that I've seen time and time again over the years that can severely compromise SEO expert performance and rankings. They typically can be bucketed into the following main categories.
Bad HTML Coding
HTML coding is the code that's behind each web page. It essentially helps to configure what is visually presented on a web page. Google is heavily reliant on the HTML code and tags that your website provides. It communicates to search engines what your page is about, what to rank it for, and how good of a user experience it is. Many sites fail to configure their HTML well and do not use important tags and use very inefficient means when structuring their code, which results in slow-loading sites and a lack of search engine rankings.
As discussed, to rank for a search phrase, you almost always need to have content that speaks to that phrase. Many marketing teams want to rank for a particular search term but lack the content. If you want to rank for a large list of search terms, you need to have content and dedicated web pages that speak to those phrases.
Sometimes websites inadvertently create multiple URLs for the same page or content. This is considered duplicate content. Duplicate content was one of the early spam techniques that websites used to increase their search presence on Google and other search engines. Google instituted a penalty to discourage this practice. Unfortunately, many well-intentioned websites accidentally create multiple URLs and are flagged for duplicate content. This can result in search engine penalties and drop your search engine rankings.
Bad URL Structures
URLs and their structures carry a lot of weight in Google's ranking algorithm. As mentioned before, be sure to have a clean URL that's short and inclusive of the search term that you'd like the page to rank for. Do not use URLs that lack these characteristics.