First is that the_field() echos the field, that’s not what you want. You need to use get_field(). Then you need to get the path to the actual file. ACF only stores the attachment ID and will return what you set it to return. You need to take what is returned and then get the actual full file path to the file and use this file path to add it to the .zip file, like the code in the example your following does.
