R Shiny selectInput that is dependent on another selectInput

You can't access inputs in the ui.R part of the app so you need to use renderUi/uiOutput to dynamically generate your selectInput.

In your ui.R you could add:

uiOutput("secondSelection")

and in your server.R:

 output$secondSelection <- renderUI({
                selectInput("User", "Date:", choices = as.character(dat5[dat5$email==input$Select,"date"]))
        })

You can also do it whout changing ui.R. At least that solution works in an App that I did. Ading this to server.R should have the same effect.

observe({
      updateSelectInput(session, "User", choices = as.character(dat5[dat5$email==input$Select, date]))
})

Though it is a useful and powerful too, I find it "cleaner" te get by without renderUI. It keeps the UI in the UI and the server in the server. But that is just a matter of taste, I suppose.