Hey @Christian21, welcome to our Community!
If I understand the situation correctly, someone shared a file, or folder with you, but you can't add it to your account, because your quota is full.
Another way for you to receive the content, would be for the sender, to send you a shared link, with view only permissions instead of a shared folder.
That way, you can download the content locally to your device, without having it added to your Dropbox storage.
I hope this helps!