If you see a "Cannot find a Nacelle reference" message, a simple solution is to perform a full content re-index.
You are most likely to observed this message in content entries, under the "fields" object. Along with this message, you'll also notice that the "type" is "NacelleReference."
Why This Occurs
This message is most likely related to webhooks. You'll see this message when a reference has not been ingested into Nacelle. The solution is to perform a full content re-index. This is because a single entry reindex only reindexes that single entry at entry depth 0; it does not reindex any child references. Re-indexing all content forces Nacelle to fetch the missing asset and therefore resolves the issue with a missing reference in the parent entry.