MCVE's should start the Shiny app when they are copied in the console. An easy way to do this is using the shinyApp function. For example:

why is my checkbox not responding?

library(shiny)

ui <- fluidPage(
  checkboxInput('checkbox', 'click me'),
  verbatimTextOutput('text')
)

server <- function(input, output, session){
  output$text <- renderText({
    isolate(input$checkbox)
  })
}

shinyApp(ui, server)

Alternatively, you can also not assign variables to ui and server.

library(shiny)

shinyApp(
  fluidPage(
    checkboxInput('checkbox', 'click me'),
    verbatimTextOutput('text')
  ),
  function(input, output, session){
    output$text <- renderText({
      isolate(input$checkbox)
    })
  }
)

shinyApp(ui, server)

In practice, shiny Apps are often very complicated and full of features that have been developed over time. More often than not, those additional details are not necessary to reproduce your issue. It is best if you skip such details when writing MCVE's.

WRONG

Why is my plot not showing?

library(shiny)
library(ggplot2)

ui <- fluidPage(
  plotOutput('plot')
)

server <- function(input, output, session){
  df <- data.frame(treatment = rep(letters[1:3], times = 3),
                   context = rep(LETTERS[1:3], each = 3),
                   effect = runif(9,0,1))
  df$treat.con <- paste(df$treatment,df$context, sep = ".")
  df$treat.con <- reorder(df$treat.con, -df$effect, )
  output$plot = renderPlot({
    myPlot <- ggplot(df, aes(x = treat.con, y = effect)) +
       geom_point() +
       facet_wrap(~context, 
                  scales = "free_x",
                  ncol = 1)
  })
}

shinyApp(ui, server)

RIGHT

Why is my Plot not showing?

library(shiny)
library(ggplot2)

ui <- fluidPage(
  plotOutput('plot')
)

server <- function(input, output, session){
  output$plot = renderPlot({
    myPlot <- ggplot(mtcars, aes(mpg, wt)) + geom_point() 
  })
}

shinyApp(ui, server)