How to display the target variable in the check box group input

shiny
,
3 
  df_drop <- reactive({
  mydata=df[,grepl("input$select_var_tar",names(df()))]   
    })

This looks like the problem. Remove the quotes so it's just

df_drop <- reactive({
  mydata = df[, grepl(input$select_var_tar, names(df()))]
})