error: expected primary-expression before ‘.‘ token ＜＜ Book.displayMessage()

The wrong is like the below picture:

The reason is Book isn't Instantiation Object, it must Instantiation that it could call function, and you need know the Code must out of cout Codes blocks[displayMessage() function include cout contents.]

The repair Codes like the below program:
Output result:

Good Luck!! I hope the answer could solve your problem.