将output$cellDATA更改为
if (!is.null(input$cell_data)) { row_col <- strsplit(input$cell_data, "-") paste0("The cell selected is in row ", row_col[[1]][2], " and column ", row_col[[1]][1]) }

library(reactable)
library(shiny)

iris = iris

ui <- fluidPage(
  reactable::reactableOutput("irisTABLE"),
  textOutput("cellDATA")
)

server <- function(input, output) {
  
  output$irisTABLE = renderReactable({
    reactable(iris,
              onClick = JS("
                function(rowInfo, colInfo, e) {
                  Shiny.setInputValue('cell_data', colInfo.id + '-' + rowInfo.index)
                }
              ")
    )
  })
  
  output$cellDATA = renderText({
    if (!is.null(input$cell_data)) {
      row_col <- strsplit(input$cell_data, "-")
      paste0("The cell selected is in row ", row_col[[1]][2], " and column ", row_col[[1]][1])
    }
  })
}

shinyApp(ui = ui, server = server)