what does "natural turnover"mean?
This was for a variety of reasons—lengthy periods of time to get needed security clearances, long-delayed White House approvals, a U.S. Senate slow to confirm nominees, natural turnover, and the challenge of persuading folks to leave the private sector for government service.

Question

Answer

this paragraph is an explanation for why there were so many vacancies in the government

"natural turnover"  refers to people just leaving their jobs in government to do other things, thus creating more vacancies. Why would people leave? Well they know they are going to be replaced so they leave when convenient for them, or they get old and tired, and just retire, i.e. this is called "natural turnover" in employment jargon