26: NamespaceNotFound (error code 26)

What does it mean?

The referenced namespace (database.collection) was not found. The specified collection or index does not exist.

Why does it happen?

A misspelled collection name, a collection not yet created, the wrong database context, or attempting a drop/index operation on a nonexistent collection.

How to fix it

Verify the collection name and database: show collections. Remember that names are case-sensitive. MongoDB creates a collection automatically on the first write; if needed, create it explicitly with db.createCollection(). For operations such as drop, check that the collection exists beforehand.