What is the ruling on a ghusl in which the areas of the neck, below the chin, and behind the ear were washed incorrectly, and must previous prayers be repeated, and what is the sin for them?

It appears the questioner is afflicted with waswasa (obsessive whispering/doubts). It is sufficient for him to generalize water over his entire body during ghusl (ritual bath) based on the predominance of his belief. If he predominantly believes he has completed the ghusl, then he has fulfilled his obligation, and his state of ritual impurity has been lifted. Doubt after completing an act of worship has no effect, and what was mentioned is merely an illusion or waswasa that should not be given attention.