I believe ArcGIS Utility Network is licensed through ArcGIS Enterprise, where your organization members must be assigned a license for the ArcGIS Utility Network user type extension. This user type extension is included in the GIS Professional Standard and GIS Professional Advanced user types and can be included as an add-on for all other user types.
It doesn't look feasible to work on Utility Network using the trial (ArcGIS Online organization), since ArcGIS Utility Network User-Type is not available for ArcGIS Online.
However, you may try working on the following ArcGIS Solutions.
Utility Network Package Tools
Explore more solutions for electric, gas, sewer, stormwater, and water distribution.
Introduction to Utilities
Think Location