Hi Konstantinos Tsirgogiannis,
Thank you for posting your query on Microsoft Q&A!
You should see any licenses / subscriptions in your Microsoft admin center - https://admin.microsoft.com/Adminportal/Home#/subscriptions
In here you can cancel and delete your subscriptions. It may take some time for the subscription to fully deprovision, 30 days or more, once this is complete you will be able to delete the tenant.
Let me know if you have any further queries, I would be happy to help.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.
Kind Regards,
Donal