Hmm. That's a bit of an odd one.
It used to be a requirement for Herblore on RS3, but it has been reworked since then. However, I hope it has been bugged in the past, leaving players unable to progress.
The only thing I can think of (other than a bug) is a missed dialogue step at the start of the quest. I'd suggest heading back to the quest start and going through the dialogue again before heading to Sanfew and doing the same with him. Unless anyone has any other ideas, a bug report would be the next option.
27-Jun-2022 14:11:50