You are probably using the wrong aggregator. You can try using the JSON “Aggregate to JSON” module.
Alternatively, you can first aggregate to ARRAY, but then you will still need to use the JSON “Transform to JSON” module with the array.
For more information about the Aggregate to JSON and Transform to JSON modules, and JSON app, see the corresponding Integrations page and the Help Centre documentation.
Hope this helps! If you are still having trouble, please provide more details.
— @samliew