尝试运行以下代码以获取rmarkdown软件包的最新版本，这应该可以解决该问题:

Try running this code to get the latest version of the rmarkdown package, which should solve the issue:

install.packages("rmarkdown", repos = "https://cran.revolutionanalytics.com")

问题是RStudio IDE会查看更新rmarkdown软件包之前拍摄的静态CRAN快照.有关更多信息，请参见以下链接: https://groups.google.com/forum /#！topic/rropen/Kgg8z6FF40I

The issue is that the RStudio IDE looks at a static CRAN snapshot that was taken before the rmarkdown package was updated. For more information, see this link: https://groups.google.com/forum/#!topic/rropen/Kgg8z6FF40I