Validation error: Incorrect template "{{c.user.id}}". ValueError: invalid literal for int() with base 10: 'user'

You may be better off opening a Support Case with the Quickbase team who can access your app. The first thing I'd confirm is that both fields, the "expert_assigned" and field referenced in Step C {{c.user.id}}, are the same type. And I'd check to see if the field help text "Specify either user ID or user email" is displayed where you are setting the User (presumably) in your Pipeline step.

Also, you can cast values to different types via Jinja, but I'm not sure that's appropriate here. For example, {{c.user.id|int}}

Hopefully that helps!