Noob question (Prodigal Perfection)

Im noob and i donk know why Prodigal Perfection (2% spell dmg per 100 max.mana) dont work

i tried to take it in game, saw it dont work and tried to do it in POB, also dont work

Sry, Im noob, pls help

PS: I DONT NEED ADVICE ABOUT MY BUILD, just explain how Prodigal Perfection works

The tool tip in game is generally not very accurate, so that doesn't mean anything.

I checked with my Bane/Soulrend toon in PoB, and PP definitely has an effect. If you are just comparing what happens when you increase your mana by a small amount via Soul Siphon, which is what you're showing here, the DPS impact is very small. If you look at what happens when you turn PP on and off, the effect is easier to notice. If you are truly getting 0 increase in your Soulrend dps when you turn on PP in PoB, something is wrong somewhere.

Cheers.

+300 MP, should be +6% dmg, but in fact +0%, why is that?

As I said, it's not 0 in PoB, but it is quite small. If I remember correctly, I think your mana went up by about 150 when you added SS on top of PP. And 3% "increased" damage (or 6% or whatever) does not translate to a 3 or 6% larger total dps number than you had before. You need to look at how the damage calculations work. The wiki describes "increased" but you really need to check PoB calcs page to full understand.