I have both Lightroom CC (classic) and Photoshop CC installed.
In Lightroom I already have a catalog setup with lots of photos.
When I open up Photoshop, selected 'LR PHOTOS' on the left side, the center of the screen says 'Your Lightroom Library is Empty', Continue to Lightroom Web and upload photos there, or install Lightroom on your desktop.
What do I need to do to make Photoshop CC acknowledge the existence of Lightroom?
That start menu only shows Images that are synced to the cloud from Lightroom (e.g. from Lightroom mobile, Lightroom CC or Synced collections in Classic.
Scroll down the page in this link for an explanation :
and this for syncing Lightroom collections