app.R

library(shiny)
ui <- fluidPage(
  sliderInput('x', "x", min = 0, max = 10, value = 0),
  actionButton('go', 'Go')
)

server <- function(input, output, session) {
  observeEvent(input$go, {
    validate(
      need(input$x > 5, message = "I'll never be seen!")
    )

    showNotification(paste0("Good job ", input$x, " > 5. Now we do something cool!" ))

  })
}
shinyApp(ui = ui, server = server)