Shiny Error in Match.Arg(Position):'Arg' Must Be Null or a Character Vector

shiny Error in match.arg(position) : 'arg' must be NULL or a character vector

It's in your call to sidebarLayout (it's in your error message trace).
If you look at ?sidebarLayout, it takes 4 arguments

function (sidebarPanel, mainPanel, position = c("left", "right"), 
    fluid = TRUE) 
{
    position <- match.arg(position)
    ...

The third argument is a string "left" or "right" being the position.
Your third argument seems to be a mainPanel and you are passing in two sidebarPanels. That's why you are getting that argument.

Shiny Error when running App: Error in match.arg(position) : 'arg' must be NULL or a character vector

I believe your issue is in your UI If you notice the sidebarLayout function has four arguments and tabPanel is not of them. You currently have sidebarPanel and mainPanel but then have another tabPanel before closing the sidebarLayout.

If you delete the tabPanel you should be able to get it to work.

You can read a little more on this other question to get a better idea of what I mean.

shiny Error in match.arg(position) : 'arg' must be NULL or a character vector

Error in match.arg(position) : 'arg' must be NULL or a character vector

There is an extra div() element in the second tabItem in tabItems in ui.R. Either provide an argument you implied to or remove that div() element. Also, I have added session argument to the shinyServer() function in server.R. After these changes app is running without any errors.

EDIT :

You forgot to add one more tabItem for Contingency in tabItems() function. Also, it is always better to differentiate between tabName and the title for that tab. And there should be no spaces in the tab name as per my experience, that is the reason why Bivariate Regression tab was not working before. It should work fine now.

Updated code:

ui.R

library(shiny)
library(shinydashboard)
library(ggvis)
sidebar <- dashboardSidebar(
  br(),
  sidebarMenu(id="tabs",
              menuItem("Import Data", tabName = "import", icon=icon("list-alt")),
              menuItem("Bivariate Regression", tabName="bivariate_regression", icon=icon("line-chart")),
              menuItem("Contingency", tabName = "contingency", icon = icon("table"))
  ))

body <- dashboardBody(
  tabItems(      
    tabItem(tabName= "import",
            sidebarLayout(
              sidebarPanel(
                fileInput("file","Upload the file"), 
                tags$hr(),
                h5(helpText("Select the table parameters below")),
                checkboxInput(inputId = 'header', label= 'Header', value= TRUE),
                checkboxInput(inputId = "stringsAsFactors", "stringsAsFactors", FALSE),
                br(),
                radioButtons(inputId = 'sep', label = 'Seperator', choices = c(Comma=',', Semicolon=';', Tab='\t', Space= ' '), selected= ',')
              ),
              mainPanel(
                uiOutput("tb")
              )
            )),

    tabItem(tabName= "bivariate_regression",
            sidebarLayout(
              #div(),
              sidebarPanel(
                fileInput('datfile', ''),
                selectInput('x', 'x:' ,'x'),
                selectInput('y', 'y:', 'y'),
                uiOutput("plot_ui")
              ),
              mainPanel(
                titlePanel("Plot Output"),
                ggvisOutput("plot")
              )
            )),
    tabItem(tabName="contingency", h2("Contigency Tab content"))
  ))

dashboardPage(
  dashboardHeader(title = "COBE Dashboard"),
  sidebar,
  body)

server.R

library(shiny)
library(dplyr)
library(ggvis)
shinyServer(function(input, output,session){   
  #read the data and give import prefrences  
  data <- reactive({
    file1 <- input$file
    if(is.null(file1)){return()}
    read.table(file=file1$datapath, sep= input$sep, header= input$header, stringsAsFactors= input$stringsAsFactors)
  })

  # display summary of table output
  output$filledf <-renderTable({
    if(is.null(data())){return ()}
    input$file
  })

  output$sum <- renderTable({
    if(is.null(data())){return ()}
    summary(data())

  })
  output$table <- renderTable({
    if(is.null(data())){return ()}
    data()

  })

  #generate tabsets when the file is loaded. 
  output$tb <- renderUI({
    if(is.null(data()))
      h2("App powered by", tags$img(src='Blue.png', height= 100, width=250))
    else
      tabsetPanel(tabPanel("About file", tableOutput("filledf")), tabPanel("Data", tableOutput("table")), tabPanel("Summary", tableOutput("sum")))
  })
  ########## Data import end #########

  ########## Bivariate regression begin ###########
  #load the data when the user inputs a file
  theData <- reactive({
    infile <- input$datfile        
    if(is.null(infile))
      return(NULL)        
    d <- read.csv(infile$datapath, header = T)
    d        
  })

  # dynamic variable names
  observe({
    data<-theData()
    updateSelectInput(session, 'x', choices = names(data))
    updateSelectInput(session, 'y', choices = names(data))

  }) # end observe

  #gets the y variable name, will be used to change the plot legends
  yVarName<-reactive({
    input$y
  })

  #gets the x variable name, will be used to change the plot legends
  xVarName<-reactive({
    input$x
  })

  #make the filteredData frame

  filteredData<-reactive({
    data<-isolate(theData())
    #if there is no input, make a dummy dataframe
    if(input$x=="x" && input$y=="y"){
      if(is.null(data)){
        data<-data.frame(x=0,y=0)
      }
    }else{
      data<-data[,c(input$x,input$y)]
      names(data)<-c("x","y")
    }
    data
  })

  #plot the ggvis plot in a reactive block so that it changes with filteredData
  vis<-reactive({
    plotData<-filteredData()
    plotData %>%
      ggvis(~x, ~y) %>%
      layer_points() %>%
      add_axis("y", title = yVarName()) %>%
      add_axis("x", title = xVarName()) %>%
      add_tooltip(function(df) format(sqrt(df$x),digits=2))
  })
  vis%>%bind_shiny("plot", "plot_ui")

  ##### add contingency table ########
  # display contingcy table output
  output$foo <- renderTable({
    if(is.null(data())){return ()}
    as.data.frame.matrix(table((data())))
  })

})

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