Opening iPhoto

Is the system version given below your question still correct "macOS 10.14" or did you upgrade to a new system version? After a system upgrade Photos may no longer be able to open your Photos Library, if you are keeping it on an unsupported external drive. If the library is n an external drive, try to set the "Ignore ownership on this volume" flag, as described here: Move your Photos library to save space on your Mac - Apple Support

If you cannot set this flag, something is wrong with the external volume.

Where on your computer is the Photos Library?

In the Pictures folder or somewhere else?

Phots may be trying to open a different library. Try to open your Photos Library by selecting it and double-clicking it. This way you can be sure, which Library Photos is trying to open.

Where is your library's stored? If on an external that needs to be formatted Mac OS X Extended, Journaled or apfs