I can not understand why you are returning this error

  

Error in round (transform (x), digits):     non-numeric argument to mathematical function

My code:

leaflet(br_munic) %>% addProviderTiles(providers$OpenStreetMap) %>% addPolygons(
      fill = T, stroke = F, fillColor = pal(br_munic@data$x),smoothFactor = 0.2, fillOpacity = 0.5, 
      label = paste0(br_munic@data$NOME_MUN, ": ", formatC(br_munic@data$x, big.mark = ","))) %>%  
      addLegend(pal = pal, values = br_munic@data$x, opacity = 1.0)
  }