This error is due to a default setting with new Simulink models, which is incompatible with the Contact Forces Library.
To resolve this:
- Go to Simulation > Model Configuration Parameters
- In the Simscape Multibody > Diagnostics pane, change the "Rigidly constrained block" diagnostic to "warning" or "none"
For those interested the details: The Contact Forces Library blocks uses "External Force and Torque" blocks to apply contact forces/torques to both bodies in contact. If at least one of these bodies is rigidly connected to the World Frame, you will get this error based on the diagnostic setting above. This error essentially warns you that you're applying a force to a body that will not move regardless of the applied force.