If that happens to you it means there is an error. First, you need to know if your internet has strong signal. Weak signal gives an error. Second, your app should be updated. Your phone will always tells you that you need to update it and you just need to click on something to get it done. Third, you should be using an app and not on Facebook on the website. It is more secure. Fourth, does your password match what you saved or what you always use? Fifth, you can try to turn your phone off and turn it on and try to log in again. Maybe you meet an error along the way. If these does not work, you can always change your password on your Facebook. Actually, regularly changing your password on your social medias could make it more secure and burden free.