Layout in R Shiny app that include R Markdown file

[mosman2]

I am facing a problem with the layout in R Shiny when I use the R Markdown file inside it I get the final result in a wired layout size (small and only in the middle of the screen ) as shown in the following photo:

Attached to you the code:

library(shiny)
library(shinydashboard)
library(knitr)

ui <- 
    dashboardPage(
      dashboardHeader(title ='Virtual Excursion'), 
      
      dashboardSidebar( sliderTextInput(
        inputId = "mySliderText", 
        label = "Story line", 
        grid = TRUE, 
        force_edges = TRUE,
        choices = c('1','2')
      )
      ),
      dashboardBody(
        fluidRow(
          column(9,  
                 box(
                   title = "Operations ", 
                   closable = FALSE, 
                   width = 9,
                   status = "primary", 
                   solidHeader = FALSE, 
                   collapsible = TRUE,
                   uiOutput("operations")
                 )
          )
        ), 
        fluidRow(
          column(9, 
                 box(
                   title = "Challenges", 
                   closable = FALSE, 
                   width = 9,
                   status = "primary", 
                   solidHeader = FALSE, 
                   collapsible = TRUE,
                   uiOutput("challenges")
                 )
          ) 
        )
      )
    )
    

server <- function(input, output,session){
  
  output$operations <- renderUI({
    req(input$mySliderText==1)
    
    HTML(markdown::markdownToHTML(knit('trial1.rmd', quiet = TRUE)))
    
  })
  
  
}

shinyApp(ui = ui, server = server)

Could you please guide me on how to fix this problem!