Using that section of the hard drive is no longer supported for sharing a library between multiple users on the same Mac. It was dropped back with Mavericks or earlier. The permissions do not get set properly for all users when it's used by one user.
That's probably why it's not able to be converted by Photos. Drag it into your Pictures folder and covert it with Photos there.
If you want to share that library between multiple users on your Mac you'll need to place it on an external hard drive that is formatted OS X Extended (journaled) with ownership set to be ignored. Then anyone can who has access to the EHD can use the library without compromising it for the other.
NOTE: that EHD can not be the same drive that is used for Time Machine.