There are many ways to make a mistake that leads to a zero pivot error, for example, in a Remote Displacement, typing a 0 in the wrong DOF and leaving the required DOF as Free, or in a General Joint, turning on the wrong constraint and leaving a required constraint out. Forgetting to use Shared Topology to connect two parts of a mesh. Forgetting to add a bonded contact.
You don't need to manually convert nonlinear contacts to linear before you run a modal analysis. Mechanical automatically changes closed frictional contacts to bonded contacts.
If you get a RBM error in Static Structural and run a Modal on that, you will see several (often six) zero frequency modes in the results table. Plot displacements for all modes solved, the color of the part that is moving is red and the rest of the structure is blue. If the part that is moving is hidden inside, look at the Details window where a line lists the component in the assembly that has the maximum displacement.