Hello Theodors Stroh as Admin,
As per the description, we understand that you are not able to verify the domain in Entra ID tenant.
We noticed that your domain is already registered in one of the Entra ID tenants.
You can find the Tenant ID on which your domain is already registered by verifying your domain at the below url
Once you get the tenant ID, please let us know if you have access to the tenant with Global Admin credentials.