Indeed, you can't pick up herbs, you have to make potions out of them by dragging empty bottles over them, HOWEVER, you have to make sure you have the right level of Alchemy.
Alchemy:
Level 1: You can mix small potions. (Try making regeneration, by mixing health and mana)
Level 2: You can make small potions. (Using small empty flasks)
Level 3: You can make normal sized potions. (Using empty flasks)
Level 4: You can make large potions. (Using empty super flasks, or whatever they're called)
Level 5: You can increase the effect of a potion by dragging them over augmentators (augmentator= special kind of herb)