Photo galleries and projects, in order to link together, must be assigned to the same user. As an admin, if you’ve created a gallery, but assigned it to the wrong user, simply “re-assign” that gallery to the correct account.
To do this, go to “Photo Galleries” and select the user that currently owns that gallery from the dropdown menu. Click the link called “re-assign” next to the gallery in question. Select the new user from the dropdown menu and submit.