I'm currently doing the extra missions for the Thieves Guild. I have this one to steal a necklace from Oengul War-Anvil in Windhelm. I've noticed that for all of his items i get about 70%, but for the necklace it's 0%. I've bought a few levels in pickpocketing from a skill trainer, invested two points in the first perk giving a supposed 40% increase in chance, and took a potion to add another 20%. Still the necklace is 0%, and the other items are 90%. What do i do? Is there perhaps a console command to take the item? Or should i use the advance quest console command?

Is the item worn?

Did you try taking it anyway? Maybe it's supposed to be 0% so it triggers a scripted event.

If not ... try a more creative solution.

I've tried taking it every time, and been detected every time. I'm not allowed to kill him, so i don't know where this leaves me.

Enchant 4x armor with pickpocket bonus. I wouldn't bother waste perks on pickpocketing, apart from 100+ capacity

If you don't have the perk that allows you to pickpocket equipped items, then you might have to get creative.

Items you cannot pickpocket don't even show up on the list in fact. More probably, the item is VERY valuable so extremely hard to pickpocket. Just give up this randomly generated quest and get a different one.