错误消息表明search_tweets（）函数中的location参数需要单个位置，但传递给函数的states_a变量是多个位置的向量。
你可以使用lapply来实现你想要的：

library(rtweet)

auth <- rtweet_app()

# Create a function called FUN
FUN <- function(i) {
  search_tweets(q = "#unemployment",location = i, n = 2000,
                                       lang = "en",
                                       include_rts = FALSE, token = auth)
}

states_a <- c("ondo", "ekiti", "ogun", "lagos", "osun", "oyo")

# Use lapply to searh for each location
unemployment_tweets_a <- lapply(states_a, FUN)

# Then use rbind to convert the list to a dataframe
unemployment_tweets_a  <- do.call(rbind, unemployment_tweets_a)

通过这种方式，您将获得来自您指定的所有6个位置的推文。