Shiny app error: $ operator not defined for this S4 class

Question

trying to make a shiny app using leaflet (sorry im a noob :( )

i keep getting this error when i run it: $ operator not defined for this S4 class but it's not an S4 class? im not sure, r coding gods please help me

my code:

ui <- dashboardPage(
  dashboardHeader(title = "Rental Data Explorer"),
  dashboardSidebar(
    selectInput("map_type", "Select Map Type", choices = c("All", 
"One Room", "Two Room", "Three Room", "Four Room", "Five Room", 
"Executive"))
  ),
  dashboardBody(
    leafletOutput("map")
  )
)

server <- function(input, output) {
  output$map <- renderLeaflet({
    data <- switch(
      input$map_type,
      "All" = merged_data_all,
      "One Room" = merged_data_one,
      "Two Room" = merged_data_two,
      "Three Room" = merged_data_three,
      "Four Room" = merged_data_four,
      "Five Room" = merged_data_five,
      "Executive" = merged_data_exec
    )

    mapview(data, zcol = "mean_monthly_rent", label = "Name", layer.name = 'Mean Rental Price  
(SGD)')
  })

}`

snippet of dataset

      Name mean_monthly_rent                       geometry
1     ANG MO KIO          3036.667 MULTIPOLYGON Z (((103.8572 ...
2          BEDOK          2962.757 MULTIPOLYGON Z (((103.9319 ...
3         BISHAN          3228.495 MULTIPOLYGON Z (((103.8492 ...