无法在RStudio中安装devtools——未找到依赖项(xml2/rversion)

I'm new to R and RStudio but trying to learn and put together a ShinyApps app. I cannot get past Step 1 of the Shinyapps process, which is to install the devtools package in RStudio. I believe the underlying cause is that the "xml2" package dependency is not installed, but I can't seem to resolve that and I don't understand what to do. I'm new to R and RStudio but trying to learn an