I cannot install Facebook and other application

Sorry, but the iPhone 6 can only be updated to iOS 12.5.7 and those apps need at least iOS 13.4

You will need a newer model to be able to install newer iOS versions and supported apps.

iOS 13 and iPadOS are compatible with these devices - Apple Support

iOS 14 and iPadOS are compatible with these devices - Apple Support

iOS 15 and iPadOS 15 are compatible with these devices - Apple Support

All of those programs require iOS 13.4 or higher to run.

Your iPhone 6 can not go higher than iOS 12.5.7. You cannot install any of those programs.

Time to buy a newer phone capable of running a higher iOS.