Body Nippel is unconstrained in the UY direction.
If you drop a Modal Analysis on this Static Structural model, add a Fixed Support to the ID of the main shaft and request 10 modes (not the default 6), you will see the offending body floating around and can take corrective action.
Did you insert a Contact Tool into the Connections folder and evaluate Initial Contact Status? All contacts must show Closed. None can be Near Open or Far Open. Increasing the Pinball Radius on those may resolve the problem.