I have an iPhone 6 but can't install version 13.0. do I need an upgrade on the phone?

Unfortunately you need a newer phone for 13 onwards

Why i cannot download a new version of a software 13.0? 😔

iOS 13 is not available for any iPhone. The current iOS version is 14.7.1.

Your iPhone 6 does not have sufficient RAM to run iOS 13 or later and you cannot add it.