Why can't I assign this backlog item to this Kanban board?

Make sure that you've provided a detailed estimate in the backlog item or its tasks. Note that you can avoid estimating each task individually in a backlog item. For more on this, see here. You can also use default estimates.

Also, if work has already started on the backlog item, the Kanban board must have columns that match the status of the backlog item and its tasks.