Very disorganized child

Before You Begin
Pause and breathe. Even if this shouldn't be happening... accept that it is and embrace it with composure and calmness.
Calmness is the only way forward because without calm, there is no possibility for receptiveness and connection. Your child won't give to you any more than they see from you.
Never forget that a strong connection is fundamental, as there will be no correction in them if there is no connection with you.
The Core Problem
Not understanding how they benefit from organization.
What Will Make It Worse
Constantly pointing out their disorganization with no other constructive conversation, and/or doing the minimum straightening up for them will just drag out the problem.
What The Child Needs
1. To understand the benefits of being organized: Discuss how being organized can make daily life easier, more efficient and less stressful. Help the child identify areas where they can improve their organizational habits.
2. Organizational skills: Demonstrate examples of/lead them in implementing practical organizational approaches that they feel good about to better manage their belongings, tasks, and responsibilities.
3. Sense of pride: Foster a sense of pride and self-confidence by acknowledging their efforts and progress in becoming more organized.
4. Support: Provide support and guidance as the child develops their own organizational habits.
How To Have The Conversation With Your Child
Find a place free of distractions where the child will feel comfortable talking freely.
Begin the conversation gently, expressing that you have noticed recently the child has been having trouble finding things when they need them, late to/missing commitments they forgot about, and/or feeling overwhelmed with what they have going on. [Provide your specific examples if you can]
Continue with empathy, expressing that you know how stressful and tiring it is when everything needs extra effort to make up for being disorganized, and that you want to help them never feel that again.
Work through the provided list of needs with them to get them from a place of problems to pride.
Ideally your child comes away from the conversation understanding that the hassles they are feeling right now can all go away, and motivated to implement the discussed quick and easy organizational changes for their benefit.