What is the correct order to enable Nonprofit Cloud in a new org, and what breaks if you get it wrong?

There is a documented prerequisite sequence and it exists because the dependencies are real.

1. Person Accounts and group memberships first — the constituent model everything else attaches to.
2. Data Processing Engine next, because the aggregation and scoring capabilities are built on it.
3. Actionable Segmentation after that, since it depends on Data Processing Engine. Enabling it first is the classic mistake, and the symptom is segments that will not produce usable results.
4. Then Interaction Summaries — including role hierarchy-based sharing for interaction summaries and interactions if notes need to be private to the owner and their management chain — followed by supporting features such as interest tags, record alerts and timeline.
5. What breaks if you get it wrong is rarely a hard error. It is usually a feature that silently does nothing, which is harder to diagnose, so I treat the sequence as a checklist rather than a suggestion.