Which version of DFFS are you using? If it is a version prior to v4.4.x, and you use the “CEWP” method, you must verify that you have the correct path to all the plugin files in the files “/SPJS/DFFS/CEWP/DFFS_frontend.CEWP.html” and “/SPJS/DFFS/CEWP/DFFS_backend.CEWP.html”.
Have you looked at the installation manual here: https://spjsblog.com/dffs/dffs-installation-manual/
Alexander