I cannot open the facebook app on my phone

1. Is this your corporate iPhone? Then please contact the system administrator of your corporate
2. If it is your personal device, in that case, do you have any profile installed by your corporate/organisation/enterprise/School Management?
   1. Erase the profile added by you If you have installed a profile on your own --> Install or remove configuration profiles on iPhone - Apple ...
   2. Or please contact the system administrator of the organisation who provided the profile to you

Did you get in touch with the app developer directly --> How to contact an app developer - Apple Support

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.