Either the “from” or “to” square is not in the range 1..32, so this move cannot be entered into the book.

The User Book is either not open, or not on at this time.  Select the "User Book" from the ‘Book’ menu before changing it.

You cannot enter any more moves for this position.

This move is already in your book for this position.

This move is not in your book, therefore it cannot be deleted.

This position is not in your book, therefore it cannot be deleted.

No move has been played, therefore there is nothing to add/delete to this position.