Can iPhone 6 use AppleCard?

Yes it can.

Take a look at this support guide and see if you're eligible, keeping in mind that, at the moment, Apple Card can be used in USA only:--> How to apply for Apple Card - Apple Support

Regards

Giulio

iPhone 6 using iOS 12.4 may use Apple Card.

How to apply for Apple Card - Apple Support

Correction, Apple Card can be OBTAINED in the US only. For those in the US who already did obtain one, the Apple card can be used internationally since it's Mastercard (and there is no fee on foreign transactions). Obtaining an Apple Card and using it are two different things.