Facebook not working on my iOS 12.5.5

Facebook requires iOS 13.4 or higher. You won't be able to use it on your iPhone 6 which can only go to iOS 12.5.5.

You can try the web version on Safari.

Facebook requires iOS 13.4 or higher..Which means you cannot install Facebook 5s or 6..at least requires 6s..(Currently run on iOS 15.6.1)

1. delete the app
2. Force reboot device - Apple Support
3. add app again

На самом деле 12.5.6, хотя и это не помогает установить FACEBOOK

You don't. If you need a higher iOS, you will need to purchase a newer phone

how can I update my iOS 12.5.5

It is not possible. iOS 12.5.5 is the end of the road.

12.5.6 was released on August 31st. When Bob wrote his post on Aug 27th, 12.5.5 was the current version.