Answer
This misleading low-level error is commonly a symptom of a completely different underlying problem — the parent Customer Agreement header record failing to import successfully first, due to unrelated separate errors on that header, meaning the agreement referenced by a subsequent part-level import record doesn't actually exist yet at all when that part-level import runs. Rather than treating this as a genuine usage-definition/dictionary problem, add an explicit existence check, calling the Customer Agreement API's Exists-style method against the target agreement ID, before attempting to import a part into that agreement, and skip or flag the part-level import when the parent agreement doesn't actually exist — this avoids triggering the confusing RmAccUsage-level error entirely for what is really just a missing-parent-record issue.