Iwas trying to use INSERT_FAMITEM to insert a part into the family table of an assembly, and SA kept erroring out without telling me specifically what the issue was. I tried all sorts of workarounds including USER_SELECT, but to no avail. I could manually add the part to the family table, but I was going nowhere with SA.
Then I realized that I had a another part that was assembled dependent to the part I was trying to add to the family table, so I reasoned that maybe I could add the dependent part to the family table first. Nope, which makes sense in the end, as the issue is not about order, but about dependency.
What I had to do was group them together, and then add the group feature to the table using INSERT_FAMITEM. Works like a charm now. Hope this helps someone