I can’t download Facebook on my iPhone 8

Here is how to: Update your iPhone or iPad - Apple Support

And what to do: If your iPhone or iPad won't update - Apple Support

Once this iPhone 8 has been updated, you should then be able to download Facebook app.

iPhone 8 is supported on iOS 16, but not on the current iOS 17.

You might have iOS 13.3.1 or older versions of iOS 13, Your iPhone 8 can be updated up to iOS 16.7.8. Update your iPhone to iOS 13.4 or recent version of iOS in order to download Facebook.