You have v5 footprints in the project. The 3D model path is embedded in the footprint. It includes the ${KISYS3DMOD} variable which is legacy from v5.
EDIT: You can define that variable in Preferences → Configure Paths. Give it the same path than KICAD6_3DMODEL_DIR. The 3D libraries are (mostly?) compatible, so you have a good chance of getting several or most of the models back. This of course depends on your design.
2nd EDIT: You can also download the old 3D library, save it somewhere and point the variable there.