Can I download Facebook on an iPhone 6?

Apple released the iPhone 6 in September 2014 – about nine and a half years ago.

That phone originally shipped with iOS 8, and can't run anything higher than iOS 12. Many modern apps require versions of iOS that the iPhone 6 cannot run. To run them, you need a more recent iPhone.

Can't download Facebook new to Iphone an only have a 6

You are correct. You can't download Facebook to an iPhone 6.

Minimum requirement would be an iPhone 7, but that will be out of date very soon. Recommend an iPhone 11 or higher.

And your “6” can only run up to iOS 12.

Facebook only works on iOS 13.4 and later on iPhone