I backed up my photo library using iPhoto a couple of years ago. Now I'm trying to view it and "photos" is the photo app on my computer now, not iPhoto. Photos won't let me open the old library. I downloaded iPhoto v9.6.1 per the forum's suggestion, but the operating system I use won't open it. I'm worried my photos are lost forever!
Are you running macOS 10.15 Catalina? You cannot run iPhoto any longer on Catalina.
Try to open the iPhoto Library in Photos for Mac.
This will only work, if your iPhoto Library is on volume, that is compatible with Photos. It must not be on a volume, that has been used for Time Machine backups or is accessed over the network. The file system format should be MacOS Extended (Journaled) or APFS.
If your iPhoto Library is still on a backup drive, copy it to your system volume, provided you have enough free storage. Or move it to a different volume, see: Move your Photos library to save space on your Mac - Apple Support
Backed up iPhoto Library not openable
