Is this in all areas of functioning, or just novel or specific situations? Kids know when something is hard for them by kg.
"i can't" often avoids a task or direction. Break the task down into parts & say "i'll help you, but i won't do it for you, " if the task is age-appropriate. Ignore the protests; wait 2-3 minutes & calmly re-state "it's time for you to.."not yes, you can!" practice!