What is the extension for this iPhoto Library? Is it "iPhoto Library.photoslibrary?" It looks like it is not a Library that has been converted to work with Photos-- the icon indicates that it will only work with iPhoto, and the iPhoto app isn't supported any more. If that's the case, you may be able to open a Photos Library and use File>Import to bring in the pictures from the iPhoto Library, but you would lose the Events organization.
How did you "migrate" this Library?
You can merge the iPhoto Library into a Photos Library, turning Events into Albums, by using the trusted 3rd party app PowerPhotos ($40.) Without an OS that will run iPhoto, there isn't another way to keep the organization.
Where is this Library located? The usual place is the Pictures folder on your internal drive. Do you have it on an external drive? That's OK, if it's properly formatted. To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:
Move your Photos library to save space on your Mac - Apple Support
If this drive is in a an incompatible format, stop running Photos with it immediately! A Photos Library can sit on an incompatible drive, but running it may corrupt the database.
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