Is private relay available in iphone 6

Directly from the website linked above:

Note: iCloud Private Relay is not available in all countries or regions and is currently in beta in iOS 15 and iPadOS 15.

Since your phone can't run iOS 15, you don't have private relay available.

iCloud Private Relay is not available in all countries or regions and is currently in beta in iOS 15 and iPadOS 15.

It is NOT available on your iPhone 6 because it is running iOS 12.x.x and cannot run a newer iOS version.

Axel F.