Problem with reinstalling Facebook app on iPhone
Here are a few things that you can try. These will be non-technical answers to you question.

Go to Settings&gt;General&gt;Date and Time and toggle the Automatic switch. This will set the correct time for your iPhone, as sometimes the App Store does not download apps to iPhones with their clock set in the future.

Restart the iPhone by holding down on the lock and home button at the same time for about 10-15 seconds until the Apple Logo appears (there is no red slider) then let go of the buttons.

Sign out of the App Store in Settings, sign in again and try to download.

Sometimes, users have connection problems to the App Store (or the App Store has an internal problem) so wait and try again later. You can use Facebook in Safari in the meanwhile.

Mainly this error occurs when Apple changes their Policies and Terms, or when the wrong time is set on the iPhone, because the iTunes/App Store Session is invalid or has expired. So this should help you, otherwise let me know in a comment and I will try to help :)