wrong version of photo library

The Photos.app is a part of the system. When you upgraded to Mojave, you upgraded to Photos 4.0. On Sierra you can only run Photos 2.0, and Photos 2.0 cannot open photo libraries that have been upgraded by Photos 4.0.

If you have an older version of your Photos Library from the time you used Sierra on your backup, restore this version. But do not overwrite your current library, you may need to recover the most recent photos from it.

If you do not have a Photos 2.0 backup of your Photos Library, it would be better to go back to Photos 4.0. You can then upload the library to iCloud Photos (Library). You may have to upgrade the iCloud Storage plan for at least a month.

When you now downgrade again to Sierra and Photos 2.0, you can download a Photos 2.0 version of your library from iCloud.Simply create a new empty library in Photos 2.0, then enable this library for iCloud Photos Library. The library you download fom iCloud will be a Photos 2.0 library. Without iCloud you cannot downgrade a Photos library to an earlier version. You can just extract all photos, but will lose your albums and titles, descriptions , keywords.

Hi

My first thought is there should be the backup of the original photo library in the same place as you found Sierra. Where did you find the Sierra?

Alternately check the 'Pictures' folder in finder I recall seeing archived library there.

Great, that you could recover the library from your backup. The only magic on a Mac is now iCloud. 😀 It keeps the data in sync and compatible between different device models and system versions, but without iCloud it is getting harder to manage Apple's ecosystem.