The confirmation from the landlord (Wohnungsgeberbestätigung) is a document you get when you move into an apartment. You need this document to register your address (the Anmeldung). Without this document, you can't register your address.

You get the Wohnungsgeberbestätigung from your landlord. The main tenant in a WG can also give you this document, but only with the permission of the landlord1, 2.

The landlord can't refuse to give you this document. The landlord can't refuse to let you register your address. If you can't get this document, it's probably because the landlord doesn't know that you live there. You can read more about it here.

The Wohnungsgeberbestätigung below is an official example for Berlin. You can use a different template, but it must have the same information1.