Why can’t I update to IOS 13

Perhaps if you could provide information regarding the device?

The existing version of iOS?

EDIT: the iPhone 6 is not compatible with iOS 13 or higher.

The highest possible for that model is iOS 2.5.5.

What phone do you have? An iPhone 6 or 6 plus cannot update past iOS 12.5.5.

Possibly because your device is not compatible with iOS 13 or higher?

As above, that is the highest possible for that model.