It is totally normal to not be able to finish a book. Whether it's because you're not interested, not in the mood to read or simply that the book is not up to your standards, not finishing a book is fine.
There are many books that I haven't been able to finish, for example Kendall and Kylie's fantasy book. I think that as bookworms, we find it hard to put down a book without having read the last page of it, but forcing yourself to read is the worst idea ever. Not only does it put you in a reading slump, you will find reading (a task that you usually enjoy) hard and burdensome.
What I suggest what you can't finish a book is to
1) Put it down for the moment
Don't give it up completely. If the reason you can't finish it is because you are not feeling the book, but still like it, then give yourself a break. Wait a few days or a week before picking it back up.
2) Watch book reviews of the book
Sometimes it's hard to continue reading a book if you think that the ending will not meet your expectations, or that you simply don't find it as good as you thought. By watching book reviews, you can see what others thought of that same book and how they interpreted some things that you haven't noticed. That way, it gives you a new understanding of the book!
3) Put it down completely
This is the hard thing to do, but if you don't like what you are reading, there is no purpose in continuing. Do not waste your precious time on a book that you don't enjoy when there are so many more out there you could be reading!
4) Pick up another book from the same author
If it isn't the writing style that is bothering you, try something else from the same author. Maybe reading something that is familiar but with a different plot line will help you understand what you like and dislike about the first book.
5) Read the book somewhere else
I know this sounds weird, but a change of environment can help you read. Sometimes, reading in your house can get boring, so bring that book to a nearby coffee shop or a local library, and read it there. Reading it in a different place might help you get into the story more!
Here were my 5 tips on what to do when you can't finish a book. Trust me, I have experienced this many times and although it is unpleasant, it happens. That's how you know if you liked a book or not!
Thanks for reading,