I can’t download iOS 12.4 ,

Are you looking to upgrade to iOS 12.4, or the latest available, 12.4.4, for your iPhone 6? Are you trying to do an "over-the-air" update or by connecting your iPhone to a Mac running iTunes? If the former, try connecting your phone to the Mac for the update.

kiumars288 wrote:

i don’t know what to do, maybe I should cancel it and start all over...

Yes, but this time do so with your iPhone connected to a computer with iTunes.