This is a common question that comes up a lot on this forum. The model is definitely unstable but tracking down how it is unstable is the real problem. I went through a quick stability diagnosis in a duplicate of your study like this:
1. Go into the study 'Properties'
Turn on 'Use soft springs to stabilize model'
Turn off 'Compute free body forces'
Turn off 'Large displacement mode'
2. Suppress all loads and apply only 'Gravity'
3. Mesh/Run the study
4. When the solver stops (if it stops) due to large displacements and prompts to put the study into 'Large Displacement mode' declined this offer. The solver will dump what results it has so far
5. Inspect the displacement plot in the results to find the unrestrained body
For any body that is unconstrained you would need to create explicit 'Contact Sets' in order to bond these unrestrained bodies to the rest of your structure.
This video (GoEngineer - Simulation: Common Errors in Simulation Webinar) and this document (GoEngineer - Simulation: Diagnose Unstable Studies) give more detail study instability.
I'l look at those.
I went back to other studies I have done in the past, and I can't run them in SW2018. It keeps complaining about no material assigned even though each body has material assigned.
Also, my models switch to wire frame when I am trying to assign contacts.
You may want to try duplicating your study then apply a stock material (like "Plain Carbon Steel") to the top level of your body list. Once you have done try running the copy of the study and see if it continues to complain of a lack of applied material. I expect that it will be able to work if you do this which would mean you would need to go back to your original study and reapply the materials to each body going down the list.
That sucks. I did apply a stock material and after defining EVERY connection (only 38 bodies) I got it to work.
I may try reassigning materials on the next project. I got the information I needed, Thanks.