Hello @Christian
You can use this link: https://admin.microsoft.com/AdminPortal/Home#/subscriptions to check if you have any expired licenses which are no longer active but still exist in you account. Access this page using your Global Administrator account, cancel and delete all the product subscriptions from this page. After all the product subscriptions are deleted, it may take maximum up to 72 hours. I personally have seen a delay of 12 hours for this change to reflect in Azure AD, after which I was able to delete the tenant.
-----------------------------------------------------------------------------------------------------------
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.