Can't select Photos library. Click Choose Library, nothing happens
That Library is showing as a .migratedlibrary file, which means it's already been 'translated' to Photos format &amp; can no longer be opened in Photos.

The contents must be in one of your other libraries in the list, or if on a remote disk, you may have to open it fro the `Other Library…' button.
By default, the new Photos Library will be in the same location as the old iPhoto Library it was imported from.

From How Photos for Mac handles iPhoto imports


  So, when Photos migrates an iPhoto library, it changes the file extension on the iPhoto library package to .migratedphotolibrary and makes a hard link to all iPhoto media assets in its own library package, using the same directory structure as iPhoto.