Error: dynamodb cannot read property 'push' of undefined

Cannot read property 'push' of undefined means your JavaScript code tried to append to a missing value. The failure happens locally; it is not a DynamoDB list-update error.

const items = response.Items ?? [];
const collected = [];
collected.push(...items);

const tags = item.tags ?? [];
if (!Array.isArray(tags)) throw new TypeError("tags must be a list");
tags.push("reviewed");

Here response is a completed Query or Scan response and item is a previously checked native JavaScript item. Initialize accumulators before the loop, await the SDK request, and check that the stored value is a List rather than a Set, Map or NULL. A non-array value may instead produce push is not a function.

Mutating an object returned by DynamoDB does not save it. Persist changes with UpdateCommand or PutCommand. For concurrent list updates, use an atomic list_append expression rather than reading, modifying and replacing a list that another writer may have changed.

See persisting list changes and AWS documentation.