how to open iPhoto in Catalina/

Photos replaced iPhoto several years ago. To use Photos with your iPhoto, Launch Photos with the option key held down. From the resulting dialog choose your iPhoto library. Photos will create a new library with your iPhoto media. your iPhoto library will not be changed.

From what system did you upgrade to Catalina? How did you do it?

Did you have your iPhoto Library on an external drive? Did you move it somewhere else? Is this a new computer or a Catalina Upgrade?

Did you try a spotlight search to look for iPhoto Library?

Upgrading to Catalina should not have moved the library.