why iPhoto isn't supported in my mac

The latest macOS has Photos instead of iPhoto.

Read here => https://www.apple.com/macos/photos/

Because it was discontinued 6 years ago. You must transition to Photos. Launch Photos with the option key held down. From the dialog, choose your iPhoto Library. Photos will create a new Library with your iPhoto Media.

Which system version is running on your Mac?

macOS 10.14 Mojave has been the last version of Mac OS supporting the older application that require 32-bit code, for example some old iLife system frameworks. Even if iPhoto itself is a 64-bit application, it is trying to call 32-bit iLife frameworks, that are no longer provided by the newer system versions. For the same reason Garageband '11, QuickTime Player 7, Aperture 3.6 can no longer run on macOS 10.15 or later.

iPhoto was discontinued 6 years ago. It is no longer developed or sold. It was replaced by the more powerful (and free) Photos.app that comes with the OS. You can migrate to that app very simply:

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point at your iPhoto Library. It will do the rest.

If you prefer not to use Photos there are several 3rd party apps that you can use. Which to choose depends on the kind of photographer you are and your budget.