I try to make guthix balance with stat restore potion, garlic and silver dust in my inventory. When i use garlic with stat restore, it says "you have no clue what sort of concoction this would make". Silver dust with stat restore - "nothing interesting happens". My herblore level is 24. Are there any additional requirements or Im doing something wrong?

I think there might be quest involved with it... check the herblore guide.

You actually need to know that you'll make a Guthix potion in order to make it. You learn that in "In aid of the Myreque".

Thanks much.