First of all, the most likely reason you got this error message is because you imported an ePub file that wasn't originally made in Book Creator. Only books made with Book Creator can be imported.
However, we do sometimes see this error message appearing when transferring your own books made in Book Creator online to Book Creator for iPad.
It seems the main reason for this is where unsupported emojis have been inserted into text boxes. So please go back to the original book, delete any of these and try again - it should work.
There were some errors
Sometimes you'll find that the book does actually import, but you get a message saying 'The book could not fully be imported'. In this scenario, we have imported everything we could successfully imported, but some content from certain pages could not be imported. In this case, go back to the online book and check those pages. You could try deleting one item at a time, then trying again, to see which is the item that cannot be imported.