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.

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

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

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