This error is happening because the item that you're trying to update with a new status is not locked by your process. Now there can be a number of reasons as to why this is happening, I am mentioning some below:
- The item was already updated
- The item ID was overwritten with some different item ID
- The previous item has been marked as an exception, and the process received a new item id, but it has not been locked. You cannot update a new item, because it has not been locked at the start of the process.
- The item has been deferred and that action unlocks the item.