Hello Patrick Lam,
Thank you for posting your query in the Microsoft Q&A Community.
You can leave the organization by following the steps described in the accepted answer in the forum
https://learn.microsoft.com/en-us/answers/questions/1101313/leave-directory-azure-portal
As regards the error AADSTS50020., kindly get more information by following this link.
Once you have remove your accounts from the organizations, you should be able to complete the process.
Let me know if further assistance is needed.
Babafemi