The biggest problem with not being able to say NO is how we have been brought up. Two-year-olds often say no to everything. Maybe the word sounds fun or is it trying out? It strikes me that even at that age, the child's wishes are ignored and he has to share everything and always listen. If a child does not learn to set boundaries at an early age, he will have problems with this all his life and most likely always have people around him manipulating and exploiting him.
With that, we can therefore say right away that the upbringing has failed. Parents should teach children to set clear boundaries, to defend themselves and their views.