@ OP - Sounds like you opted to create the contact surfaces manually, which is fine. You can also have Creo create them for you, along with measures, etc. by selecting "Detect Contacts" from the pulldown instead of "Interface".
You need to make sure that your parts are "contact" and not free. Originates from what interface you are in. The deafault behavior of Simulate is to go into "bonded" interface (Home --> model setup). Look in the lower left corner of your screen, it should say what mode you are in. In bonded mode all assembly parts that are in contact are considered bonded together unless you otherwise tell Creo that they are free or in contact. So by telling Creo to make them free, they are no longer bonded but also cannot come in contact with each other; hence Creo thinks that the bolt is unconstrained. Long winded explaination just to say that you just need to change the connection to contact if you are in bonded interface.
As for singularities, this can be a result of many things. But in this case, it can be the result of missing a surface on part that you made infinitely stiff.....a change from infinitely stiff to normal in the same part is an infinite jump and will create a singularity...look for stress gradients in parts that should be infitely stiff, there shouldn't be any. Verify the material properties. Sharp re-entrant corners, add a small radius where there is high stress. Your part could be failing, large deformations, etc. Then there are mesh refinements/adjustments but I won't go there for now.
@ Jonathan - From what I've seen, infinitely stiff parts can help to isolate the results when comparing from one software system to another....such as a group effort with multiple users and softwares...more apples to apples. Academic in nature....like a classroom/presentation setting, collaboration, exercise, or contest. That's my guess anyhow.