My iphone X is dead, I want to transfer my recent back up to my old iphone 6 to retrieve some important data. However, when trying to do so I get the error : "the backup can not be restored to this iPhone because the software is too old.."

My iphone X is dead, I want to transfer my recent back up from my computer to my old iphone 6 to retrieve some important data. However, when trying to do so I get the error : "the backup can not be restored to this iPhone because the software on the iPhone is too old.."

I followed https://support.apple.com/en-us/HT203434 to the letter, my software is up to date, I confirmed it from settings as well as on itunes. (iOS 12.5.3)

What should I do?

Thank you.