twListToDf function not found in TwitteR Package (R)

I would like to enquire why my program couldn't work as I have manually installed the twitteR package.

library(twitteR)
setup_twitter_oauth(api_key, api_secret, access_token, access_secret)
no_of_tweets = 100

Reuters_tweets <-searchTwitter("@Reuters",n= no_of_tweets, lang="en")
#Reuters_tweets

Microsoft_tweets <- searchTwitter("Microsoft", n = no_of_tweets, lang="en")
#Microsoft_tweets

Reuters_df <- twListToDf(Reuters_tweets)
Reuters_df

Error output: Error in twListToDf(Reuters_tweets) : could not find function "twListToDf"

Thank you.