Cannot open Facebook and WhatsApp after updating to iOS 11.2.6
I would consider doing a wipe and restore.

Basically you backup the phone, then go into Setting > General > Reset and "Erase all content and settings."

Once your phone has wiped itself and is at the setup screen, plug it back into your computer, run iTunes and restore the phone from the backup you just made.

That should allow you to then install and use apps that are currently broken.