Arelith

As the topic says, it does not seem that light pick is recognized as a rogue proficiency weapon. Thus it can not be taken as Weapon of Choice for Weapon Master. I tried this on drow rogue / assassin in PGCC server.

Light Pick could be chosen as a Weapon of Choice when trying with fighter base.

Does your character have Weapon Focus (One Handed Edged) on the rogue? That provides the Weapon Focus for the Light Pick.

Yes I had that feat on me.

As you can see, the rogue proficiency feat description show light pick as a rogue proficiency weapon. You can also see the Weapon Focus One-Handed Edged feat in the feat list above it. But when trying to select a Weapon of Choice, it does not show light pick as an option.