Check the location of the zero-volume element in your model. If it is in a crucial part clean up your geometry and turn on the "Aggressive mechanical shape checking ON" for the mesher. Try to clean up the geometry and mesh it with better quality. Check whether the shape check is "ON" or "OFF". If it is "ON" turn it to "OFF" by adding a command: "SHPP OFF" through the command snippet. If it works run your simulation and check the the stress output at that location where the zero volume element was. If the part is not crucial you can work with that solution. But judge the solution properly. Tetrahedral mesh fills the pyramid space to grow tetrahedral from the pyramid surface and at times it selects the wrong node to create the tetrahedral mesh so we get a negative or flat element or element with skewness of 1 or an element with zero volume. I suggest you clear your mesh and try to do the meshing with the "high smoothing" option in the "sizing" under "mesh" in the workbench. This may help you to avoid the zero-volume elements. Try to mesh with hex mesh dominant because the tet element sometimes causes inaccurate results as they degenerate from the hex element. But avoid meshing thin structures with hex elements.