You need to relink the folders in Lightroom with their new location. This is done by right or control clicking on the topmost folder and selecting "Update Folder Location"
So I right clicked/control on the top folder and this is the list of options that populated
Find Missing Folder...
Hide This Parent Folder
Show Parent Folder
Export this folder as as a Catalog
You want "Find Missing Folder"
I have tried that and the catalog is not available to select It is in light grey, not illuminated.
You don't select the catalog from this menu, and you don't select photos, you select the folder that contains your photos.