Adding a Document to a Basic Board

My understanding is that (currently) boards have no file storage location of their own, and documents can only be stored in projects. So as @skyehansen said, if you have a connected card, you can add a document (since the file is stored in the project). But if you have a non-connected card, then there's no place to store the document, so you can't attach a document.

I'll point you to my own similar question last summer: Kanban Boards and documents.

I can add a document to a connected card. I don’t think you can add it to the imaginary cards though.