selectInput() in Shiny R not returning any values

Question

I am trying to develop a Shiny app, a simple one. My intention at this point is to create a table and filter that table by various inputs. Right now this is my code:

library("shiny")
library("gapminder")
library("ggplot2")
library("colourpicker")
library("plotly")

ui <- fluidPage(
  h1("Demo"),
  sliderInput(inputId = "valor", label = "Rango ",
              min = min(data$value), max = max(data$value),
              value = c(min(data$value), max(data$value))),
  selectInput(inputId = "opc", label = "Measurements", choices = levels(data$measurement)),
  
  tableOutput("table")
)

server <- function(input, output) {
  output$table <- renderTable({
    data <- data
    data <- subset(
      data,
      value >= input$valor[1] & value <= input$valor[2]
    )
    data <- subset(
      data,
      measurement == input$opc
    )
    data
  })
}

levels(data$measurement)

shinyApp(ui, server)

as you can see, very simple. However, this code returns the table empty and the selectInput with no options of selection. However, if I put the values of the column by hand, the code works fine!

selectInput(inputId = "opc", label = "Measurements", choices = c("heart_rate","oxygen_saturation")),

The code above works great, the table suddenly displays data again and it filters correctly. I just don't get it! The sliderInput works great as well. The data set has been included as an enviroment variable.

This are the two different outputs (first picture with written values, second picture using levels(data$measurement)):

Why is this happening to me?!