Hello , is it possible to have random velocities when Arp is on ? Or change each note velocity when using arpeggiator ? Please help me 
Not directly using the arpeggiator, no.
I think the best you can do is have a random S&H LFO on something like amp volume. That would give you something akin to your random velocity.
If you needed something more predictable you could use another LFO shape to, for example, make the amp volume alternate between two values, giving an accent > ghost note > accent > ghost note type of thing. Or using a ramp LFO shape to make the amp volume rise/fall over a given time.
Either way, you’re programming that into the sound, and it would behave the same way whether that sounds was triggered by the arpeggiator or regular trigs. The arp doesn’t have anything to control velocity (or any other parameter except pitch) at a note (arp step) level.
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