The error message suggests that there is no column nconcentration in datacitalopram. Perhaps you could use citaloprampodsebou if that's better? I don't know what your data actually looks like.
Anyway, you can't pivot the data.frame if it contains nconcentration and aconcentration, so you'll need to ignore these columns as you did above:
pivot_longer(citalopram, -c(nconcentration, aconcentration), names_to = "nominal")
So if you have a data.frame already without these columns, that's good too.