mutate() and case_when() function

Hello All - finally I was able to successfully run the code sent by FJCC (Thanks a lot) and got my desired dataset.

However, I have a question:

I was running this code:

visadsl <- vis |> filter(ACTARMCD %in% c("VEDOIV_VEDOSC", "VEDOIV_PSC")) |>
mutate(trtn = case_when(
TRUE ~ NA #This line shouldn't be necessary because actarmcd should have only two values
)) |>
select(USUBJID, trtn)

and was getting this error:

Error in mutate():
! Problem while computing trtn = case_when(...).
Caused by error in case_when():

I read that both mutate() and case_when() functions are available in dplyr package whick is a part of tidyverse package. But the code only ran successfully when I installed tidyverse package.

Any idea what could be the reason?