How to coerce an `integer` to a `data.frame`?

Question

I want a column of one table to be a sumproduct of two columns of another table as given in the following script. When I run the code in RStudio, the following error message is popping up: "Error in as: no method or default for coercing 'integer' to 'data.frame' ".

Can someone explain and amend, what is wrong in the script?

    library(shiny)
    library(shinydashboard)
    library(rhandsontable)
    library(data.table)
    library(dplyr)

    DF1 = data.table(
        "Batch"= as.character(),
        "Amount of goods"= as.numeric(0),
        "Unit cost of goods"= as.numeric(0),
        stringsAsFactors=FALSE)

    DF2= data.table(
        "Cost of goods available for sale" = as.numeric(0),
        "Cost of goods sold" = as.numeric(0),
        stringsAsFactors=FALSE)

    ui <- dashboardPage(
      dashboardHeader(title = "Accounting"),
      dashboardSidebar(
        menuItem("Home", tabName = "home"),
        menuItem("Measurement", tabName = "measurement", 
              menuSubItem("Inventory", tabName = "table1")
        )
      ),
      dashboardBody(
        tabItems(
          tabItem(
            tabName = "table1",
            column(
              "DF1",
          width = 12,
              rHandsontableOutput("Table1")
            ),
            column(
              "DF2",
          width = 12,
              rHandsontableOutput("Table2")
            )
          )
        )
      )
    )

    server <- function(input, output, session) {

      data <- reactiveValues()

      observe({
        input$recalc
        data$df1<- as.data.frame(DF1)
        data$df2 <- as.data.frame(DF2)
      })

      observe({
        if(!is.null(input$Table1))
          data$df1<- hot_to_r(input$Table1)
      })

      observe({
        if(!is.null(input$Table2))
          data$df2 <- hot_to_r(input$Table2)
      })

    sumProduct1<-reactive({data$df1%>%
               summarize(sum(`Amount of goods` * `Unit cost of goods`))}) 

    observe({data$df2$`Cost of goods available for sale` <- sumProduct1()})

    output$Table1 <- renderRHandsontable({
        rhandsontable(data$df1, stretchH = "all") |> 
          hot_cols(colWidths = c(300, 150, 150))
    })

    output$Table2 <- renderRHandsontable({
       rhandsontable(data$df2, stretchH = "all") |> 
          hot_cols(colWidths = c(150, 150))
    })
   }
   
   shinyApp(ui, server)

Answer 1

The sumProduct1() reactive expression returns a summarized data frame with one column containing the sumproduct. In your observe function, you're trying to assign this data frame directly to a single column of data$df2:

Change:

 observe({data$df2$`Cost of goods available for sale` <- sumProduct1()})

with:

  observe({
    if(!is.null(sumProduct1())){
      data$df2$`Cost of goods available for sale` <- sumProduct1()[[1]]
    }
  })