If a sketch feature (design step) is highlighted with a red cross in the 3D History, an error occurred. Then a concrete error message is displayed:

In order to be able to see the error in text form, click on the option Show error... in the context menu.

Example 1: Error in base extrude

A negative value had been chosen for the extrusion length.

Example 2: Error in sketch

Edges cut or intersect themselves.