what to do with these sensations,these happens all day,why i have to resist to itch i am wondering when i can simply itch that move on.
The point is not to resist the urge. The meditation is in observing the effect the itch has on the mind. Satipatthana meditation is defined with three qualities: Ardency, Alertness, and Mindfulness. Let us focus on the first quality here. Though note that all three apply.
"Ardency means being intent on what you're doing, trying your best to do it skillfully. This doesn't mean that you have to keep straining and sweating all the time, just that you're continuous in developing skillful habits and abandoning unskillful ones." - Thanissaro Bhikku
So in this context, we would want to itch in a skillfull way.
How does "Simply itching and moving on" effect us? How does giving into our urges when they come about serve us?
Does giving into the minds urges immediately lead you towards the goal or away from it? Each moment you are conditioning your mind (your kamma). Conditioning your mind to reactivity can lead to unskillful qualities if not done with headfulness.
I encourage you to scratch the itch and observe how your mind reacts. Then don't scratch it and observe it. Does there need to be discomfort because of the itch.
Recall the Five Subjects of Frequent Recollection:
Kammassakomhi kamma-dāyādo kamma-yoni kamma-bandhu kamma-paṭisaraṇo.
I am the owner of my actions, heir to my actions, born of my actions, related through my actions, and live dependent on my actions.
Yaṁ kammaṁ karissāmi kalyāṇaṁ vā pāpakaṁ vā tassa dāyādo bhavissāmi.
Whatever I do, for good or for evil, to that will I fall heir.
Evaṁ amhehi abhiṇhaṁ paccavekkhitabbaṁ.
We should often reflect on this.
It's not normal to itch all day and it is a skin issue, possibly an allergy or an intolerance of some sort but can also be a liver or a kidney problem and whatnot.
If scratching an itch gives you relief from the uncomfort then you can make yourself comfortable. The Buddha adjusted the posture when his back hurt and took a nap in daytime when he saw it fit.
Medical procedures are of course allowed and the relief from physical illness is in a qualified sense a stilling of suffering.
I guess you got the idea of not scratching an itch from some 'vipassana meditation' teaching of just observing feelings and perceptions.
However if we just passively observe everything without acting on our knowledge for the purpose of stilling that which is suffering and avoiding states connected with aversion, greed and delusion then we wouldn't be going to an empty building or the root of a tree to meditate nor would we be guarding our senses.
Physical discomfort is a cause for aversion and it is difficult to think clearly when one is upset. Some defilements are to be abandoned by forbearance and some defilements are to be abandoned by avoidance, forbearance isn't always the way to deal with discomfort; if forbearance was always the answer would we eat to keep hunger away?
One doesn't have to choose between being mindful and scratching an itch because the two aren't antagonistic to one another.