How did you upgrade to High Sierra? Did you wipe your drive for a clean install and need to restore your iPhoto Library from a backup drive?
Do you perhaps have a different copy of the library on another drive?
Which version of iPhoto is currently in your Applications folder? is it any version of iPhoto '11? Version 9.x.x?
If it is an iPhoto '11 version, contact Apple Support and ask for a redeem code for iPhoto, so you can download iPhoto 9.6.1 from the AppStore. You really need it to be able to repair your iPhoto Library to prepare it for the migration to Photos.
Explain to Apple Support, that the iPhoto library needs to be repaired in iPhoto, because Photos cannot open it.
If there is no chance to obtain the update to iPhoto 9.6.1, even with the help of Apple Support,you can extract the photos from the iPhoto library by opening the package contents of the iPhoto Library:
- Select the iPhoto Library in the Finder and ctrl-click it or right-click it.
- A Finder window will open and show you the contents of the library. It will depend on the version of iPhoto, how the folders with the images inside the library bundle are named. Look for folders named "Masters" with the original masters files, "Previews" with the previews of the edited versions. In older versions of the library there could be folders name "Original" or "Modified".
You can copy these folders with originals or edited versions out of the iPhoto Library and import them to Photos.