您需要给予sidebarMenu和id参数，以便将其用作可在conditionalPanel中使用的input：

library(shiny)
library(bs4Dash)

ui <- bs4Dash::dashboardPage(
  
  dashboardHeader(title = "Simple Dashboard"),
  
  dashboardSidebar(
    
    sidebarMenu(
      id = "tabs", # <- This `id` is important
      menuItem("Tab 1", tabName = "one"),
      menuItem("Tab 2", tabName = "two")
    ),
    
    conditionalPanel("input.tabs === 'two'", # <- we use the `id` here!
      selectInput("variable", "Variable:",
                c("Cylinders" = "cyl",
                  "Transmission" = "am",
                  "Gears" = "gear")))
    
  ),
  
  dashboardBody(
    
    tabItems(
      
      tabItem("one",
              "This is the content of Tab 1."
               ),
      
      tabItem("two",
              "This is the content of Tab 2."
              )
    )
  )
)

server <- function(input, output) {
  
}

shinyApp(ui, server)