I'm trying to apply an asset package we received from one of our customers and I'm getting the error "The Utility Network must be version 5". I've just been back through and upgraded everything (ArcGIS Pro 2.7, Untools, and upgraded the Enterprise Geodatabase) but even after I stage a new utility network it looks like it's version 3. When I try to apply the asset package we received from our customer we get the error again.
The Utility Network must be version 5
Not sure if this is your problem, but worth a try...
Starting at ArcGIS Pro 2.7 for enterprise geodatabases, the tools to create and upgrade a utility network look at the ArcGIS Pro and ArcGIS Enterprise versions to determine what version of the UN to create or upgrade. You won't be able to upgrade to UN version 5 unless you have a federated server of 10.9 for enterprise geodatabases.
Check out the section on ArcGIS compatibility
You probably need to run Upgrade Asset Package tool to bring the version of the asset package to match the version of ArcGIS Pro. See https://solutions.arcgis.com/utilities/help/utility-network-automation/tool-reference/upgrade-asset-...
Otherwise, temporarily downgrade your ArcGIS Pro to v2.5.x to match UN Asset Package version you have and build UN and publish, provided you have matching ArcGIS Enterprise (10.7 or later as per the table at https://pro.arcgis.com/en/pro-app/latest/help/data/utility-network/utility-network-dataset-administr... )