Export the images from Lightroom to your hard drive as TIF or JPG and you should be able to see them. Never worked with FCP but it should be able to look on the drive itself for images so you don't have to go through Apple's Photos app. But I could be wrong. Apple is very capable of screwing their users.
Lightroom is a Database program. It does not touch your images until you Export them. LR saves edit done on images to either the LR catalog file or to an XMP sidecar file or to the file itself with DNG files. Those edits are only seen by other programs after it is Exported.
Images are not directly accessible through LR by other programs.
Lightroom contains no images files and is a cross platform database.... Photos and Aperture are Mac only just as Final Cut is... You need to locate the files on disk but they will not have the changes made in Lightroom reflected most likely so I would suggest exporting with Lightroom adjustments included copies to a specific folder for Final Cut use..
Simply put the apps aren't compatible...