It is a matter of flexing and testing and some troubleshooting. What I always do is checking the errors and see if that gives me a clue.ALs accepting to remomove he constraint sometimes helps to find the issue.
If it does not then I start with some troubleshooting. Often I will delete parts of the family (e.g some extrusions) and see if/when it starts flexing properly again (undo to return to previous situation). When I find the culprit(s) I start to remodel them or model them in a slightly different way.
In your case it seems the front part of the plinth are causing the issues (see screenshot). After deleting it, it will start to flex properly again (at least based on some quick tests).

So the solution, delete this part and create it again.
Louis

Please mention Revit version, especially when uploading Revit files.