The friction slip velocity is no longer defined as a dependent variable. When running a model using the COMSOL API, you will need to remove the reference to this variable in the solver settings. The friction slip velocity is usually defined as <comp>_<solid>_vslip_<pairname>, where <comp> is the tag of the component, <solid> is the tag of the Solid Mechanics physics interface where the Friction node is defined and <pairname> is the name of the contact pair.