Each version of Premiere Elements only works with the Organizer associated with that version. If you installed Premiere Elements 2019, it automatically installs the Organizer for that version. When you open the 2019 Organizer, it will ask you whether you want to convert the catalog from the earlier version. You can then use the converted 2019 catalog to work together with Premiere Elements 2019.
(While it is possible to add an additional Editor in the Preferences dialog of the Organizer, I just found that this only allows you to send images from the 2018 Organizer to the 2019 Elements Video Editor.)
I should add that it is also possible to click and drag media from the 2018 Organizer to the 2019 Premiere Elements. At least, it is possible in Windows - I have no idea about the Mac.