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 blocksdisplayMessage() function include cout contents.

The repair Codes like the below program:
Output result:

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