It can be multiple reasons... the model is not fully constrained, or the geometry can't mesh properly. Tell us a little more details ... what creo version you use, general understanding of the model, constraints, loads, etc... I had similar issues with cast models.
check element 1036656 as it is "corrupted" invalid geometry. The meshing step didn't work and the element may well be too distorted. Might be nothing do to with a modelled not being fully constrained !