The last system version to support iPhoto is macOS 10.14 Mojave.
The upgrade to Catalina removed older 32-bit frameworks from the system, that are needed by iPhoto. It is simply not possible to run a full version of iPhoto on Catalina, because of the lacking support for oder frameworks.
Use iPhoto on your High Sierra Mac, as long as this older Mac will survive, but any new Mac will come with a system version, where you can no longer use the older 32-bit applications. Try to find a replacement for iPhoto while you still can run it; that will make the migration easier. When you have to do the migration, when you cannot run iPhoto any longer at all and cannot see the old albums, titles, captions, keywords, it will be hard to recreate your photo libraries in a different application.
The most seamless migration is from iPhoto to Photos for Mac, because the Photos.app can open your iPhoto Libraries and transform them into Photos Libraries.
I wrote this user tip several years ago, when Photos for Mac has been released as the successor to iPhoto: Notes on Migrating an iPhoto Library from… - Apple Community. Some of the links may be broken, because Apple Support has removed most of the support documents for the migration from iPhoto to Photos - I found some, but not all on the wayback machine. I am sorry to say, but iPhoto is now really yesterday's snow, and many of the support documents are gone as irrelevant for the current system versions.
