This reprex works fine for me:
library(shiny)
ui <- fluidPage(
selectInput("label", "Select Label:", choices = letters)
)
server <- function(input, output, session) {
observeEvent(input$label, {
message("Label changed to ", input$label)
})
}
shinyApp(ui, server)
I suspect your problem is that you only have one value, so that the value, and hence input$label, never changes.