I can't use my iphoto

iPhoto was discontinued over 6 years ago. On a Mac running Catalina or later hold down the option key while opening the "Photos" app and it will give you the option to open your iPhotos library. It will convert it and open in Photos.

The last system version to support iPhoto has been macOS 10.14. iPhoto is needing some frameworks with 32bit code, that are no longer provided by the system since macOS 10.15 Catalina. All system versions since macOS 10.15 are 64-bit only. The Photos.app is the successor to iPhoto and is coming pre-installed as a part of the system.