Hi
I have solved this issue by using stringi package.
here is the script that i have used.
library(twitteR)
library(stringi)
tweets <- searchTwitter(search_term, number_of_tweets )
#convert received data to something PowerBi understands
df_tweets <- twListToDF(tweets)
#column clean
df_tweets$text <- stri_encode(df_tweets$text, "", "UTF-8")
It cleans the column which are not understandable.