If you unable to log in to your Facebook account, there must be something wrong somewhere, which might be unknown to you. Because normally, you should be able to log in into your account if all things are equal. First of all, ensure your username and password are correct. The next thing you need to do is to check your data network if the fault might be from that end, because often time, one might be unable to log in into his/her account if the data network is not strong enough to allow it.
If everything is okay with data network and it hasn't logged in, you can check if the application is running on its current update. You can do this by visiting your google play store and check if its among the applications needed for update, if it is,you can get it updated. After which, you can try to log in again.
Lastly, if this also failed, clear the application cache and the background data, and off and on your phone, and try again. I am very certain one out of all these steps would work.