What is the JSON url? Your post contains very few details about the source data and the error you are encountering. Pro does not yet support direct consumption of GeoJSON, which is very unfortunate, however, your problem may be solved by one of the many comments in https://community.esri.com/ideas/13080-arcgis-pro-direct-geojson-support
Liliana, the Data Interoperability extension supports the creation of an ETL tool to import arbitrary JSON from file or URL, this brings access to the database into the geoprocessing environment.
Dear Bruce Harold, can ETL tool convert JSON data just from nested node to FGDB table? I am not able to find appropriate tool in DI.
When using Attribute Creator tool, the outup fields are empty, or at best there is just one row. It seems like there is no iteartion.
You might have to take a couple of approaches. Just using a JSON reader does not unpack the properties. If the JSON contains an array then you may need to use a ListExploder to bring properties to top level or one of the JSON-based transformers to perform a query:
As JSON is a semantic format then handling what is in it needs to be custom. I attach an example FMW source where a CKAN JSON feed is turned into a feature service by way of example.
If you need help with this send me an email firstname.lastname@example.org.