cannot update my iPhone 6s Plus to version 13.0

Hello. This depends on what iPhone you have. Do you have an iPhone 6 Plus? Or an iPhone 6s Plus? Your question subject says "iPhone 6S" while your question category says "iPhone 6 Plus." The iPhone 6 Plus and iPhone 6s Plus are two different phone models and which one you have will make a difference in the latest version of iOS you can run.

If you have an iPhone 6s Plus, you will be able to upgrade it to iOS 15, which is the latest version of iOS. iOS 13 is no longer available and you cannot upgrade to this version. You will need to skip iOS 13 and go to iOS 15.

If you have an iPhone 6 Plus, however, you cannot upgrade beyond iOS 12.5.5.

If you are unsure of what iPhone you have, you can use the guide here to help identify it.

Thanks!

Hello. It sounds like you have an older iPhone 6 Plus. The iPhone 6 cannot run any version of iOS newer than iOS 12.5.5. If you would like to run a modern version of iOS, such as iOS 15, you will need to upgrade to a newer iPhone.

Yes the 6S plus is compatible with iOS 15 (not iOS 13, however). But a 6 Plus is not, and a 6 Plus is not a 6S Plus. The 6S Plus is one year newer than the 6 Plus.