can i cast Protective Bubble on creature who already has shroud?

[what]

is it alright to cast Protective Bubble on a creature that has shroud already, just to make it unblockable?

[Kenji]

No. A creature with shroud cannot be the target of spells or abilities. Aura spells target when they are cast, so you can't target a creature with shroud when casting Protective Bubble.

On the other hand, if you're able to get Protective Bubble onto the battlefield without casting it, then it can be attached to a creature with shroud -- this is because Auras only target as they're cast.

[what]

what about other way round could i cast the bubble on creature that is already unblockable just to give it shroud?

[Arix]

Yes.

[what]

what about casting two protective bubbles on same creature?

[Arix]

That's no different to the above situation.

[what]

which above situation because there were two and one said yes and one said no?

[Arix]

No, they're both yes (as long as you can find a way to get the Bubble on something with shroud).