reorder的正确语法（在ggplot2的上下文中）是(aes(x = reorder(x, -y), y = y)或(aes(x = reorder(x, y), y = y)，这取决于您希望从y还是最小值开始。
此外，请确保fill = "blue"，除非BLUE是另一个对应于真实的颜色的指定矢量。

library(tidyverse)
set.seed(123)

Country <- c("USA", "UK", "ESP", "FR", "ITA", "BRA", "JAP", "CHN", "ZAF", "DEU")
Participations <- sample(1:10, size = 10, replace = TRUE)
data <- data.frame(Country, Participations)

ggplot(data) + 
  geom_col(aes(x = reorder(Country, -Participations),
               y = Participations), fill = "blue", width = 0.6) +
  xlab("Country")