Error: DynamoDB ValidationException: Query condition missed key schema element

Query condition missed key schema element means your key condition omits a required partition-key attribute for the table or index being queried. Put that attribute in KeyConditionExpression with an equality comparison; a filter does not satisfy the requirement.

1. Read the missing attribute named in the exception and compare it with DescribeTable.
2. If querying an index, supply its exact IndexName. Without it, DynamoDB evaluates the base table key schema.
3. Define every #name alias and :value placeholder used in the expression. Check case and value types.
4. For a conventional single-attribute partition key, use equality on that key and optionally a supported sort-key condition. For a multi-attribute GSI partition key, provide equality conditions for all its partition-key attributes.

Suppose the table partition key is index, while EmailIndex uses email. Query the index explicitly:

import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
import { DynamoDBDocumentClient, QueryCommand } from "@aws-sdk/lib-dynamodb";

const db = DynamoDBDocumentClient.from(new DynamoDBClient({}));

const result = await db.send(new QueryCommand({
  TableName: "Users",
  IndexName: "EmailIndex",
  KeyConditionExpression: "#email = :email",
  ExpressionAttributeNames: { "#email": "email" },
  ExpressionAttributeValues: { ":email": "reader@example.com" },
}));
console.log(result.Items);

A query on a non-key field needs an index designed for that access pattern. Scan with a filter is an alternative for occasional inspection, but it reads items before filtering and can be expensive on a large table. The query builder can help construct the request.

See query examples and AWS documentation.