"message": "No value found under this key."
This error message means you are trying to access a data storage value under a key that doesn't exist.
In this situation, the storage-17 step is attempting to "append to list" but the list "items" doesn't exist yet. The builder has set the scope of the data storage step to "current run" which means the data storage step is only looking for keys that exist within the current execution (as opposed to the lifetime of the workflow or account-level scopes):
If you want to create the list if it doesn't exist yet, you can select this option in the properties panel of a data storage step:
Or, if "items" is a list that should persist across the lifetime of the workflow or persist as account-level storage, you can change the scope here: