flexdashboard错误 - pandoc：无法获取

Question

我正在尝试按http://rmarkdown.rstudio.com/flexdashboard/using.html#css_styles中所述包含css 到我的flex仪表板。

---
title: "test"
output: 
  flexdashboard::flex_dashboard:
    orientation: columns
    vertical_layout: fill
    css: https://bootswatch.com/4/litera/bootstrap.css
---

我收到错误：

pandoc: Could not fetch https://bootswatch.com/4/litera/bootstrap.css
HttpExceptionRequest Request {
  host                 = "bootswatch.com"
  port                 = 443
  secure               = True
  requestHeaders       = []
  path                 = "/4/litera/bootstrap.css"
  queryString          = ""
  method               = "GET"
  proxy                = Nothing
  rawBody              = False
  redirectCount        = 10
  responseTimeout      = ResponseTimeoutDefault
  requestVersion       = HTTP/1.1
}
 (InternalException (HandshakeFailed Error_EOF))
Error: pandoc document conversion failed with error 67
In addition: There were 13 warnings (use warnings() to see them)
Execution halted

当试图安装pandoc时，它给我这个：

> install.packages("pandoc")
Warning in install.packages :
  package ‘pandoc’ is not available (for R version 3.4.1)

有没有解决方案？