How to create an new variable based on a string of another variable

I want to create a new variable: R_T_R with levels (E_L, NER, and OR) based on another variable ID_number which has either has NR#somenumber, NER#somenumber and OR#somenumber. I have used the grepl function to identify when ID_number has either NR, NER or OR in it. See code below:

   Merge_dataset$R_T_R  <- ifelse(grepl("NR", Merge_dateset$ID_number, ignore.case =T ), "E_L",
                                                                                  ifelse(grepl("NER", Merge_dataset$ID_number, ignore.case =T ), "NER"))
                                                                                         ifelse(grepl("OR", Merge_dataset$ID_number, ignore.case = T), "OR")))

I get the following error:

Error in grepl("NR", Merge_dateset$ID_number, ignore.cases = T) : unused argument (ignore.cases = T).

Not sure who to do. When I remove the ignore cases, I get a different error. Please help. Thank you.