What stage emphasizes that children were unable yet to perform mental operations?

The stage that emphasizes children's inability to perform mental operations is the preoperational stage, according to Jean Piaget's theory of cognitive development. This stage typically occurs between ages 2 and 7, during which children exhibit symbolic thinking and language development but struggle with logical reasoning and understanding the concept of conservation. They often rely on intuition rather than logic, making it difficult for them to perform mental operations such as categorization or reversibility.