When you finish writing a book on Wattpad it's good to let your Readers Know that 'this Story is Completed'...

... - to let them know about that - you should go to Your book Settings by clicking 'My Works' on the left side of the screen & opening the whole list - then clicking on the given book Cover to open the book Settings.

Whe you do that - find the word 'Completed' at the bottom of the Screen in your book Settings & click on the white space on the right side of that word to move the marking from 'Not Completed' to 'Completed' & Click 'Save'. 

If everything goes well - your book should be marked as 'Completed' after that - which will be visible when you click on it - as if you wanted to start Reading :)

- & Everyone who starts reading your book will also see that it's 'Completed'