It looks like you don't have a rule in place that allows you to add the device as a content of the container (help here: https://pro.arcgis.com/en/pro-app/help/data/utility-network/containment.htm)
You can export all the rules in your UN project to a CSV file using the Geoprocessing Tool called: Export Rules (help here: https://pro.arcgis.com/en/pro-app/tool-reference/utility-networks/export-rules.htm). You can also check all the rules checking the properties of your UN layer, but for me the CSV file is more convenient.
If there is not a containment rule for the features you are working with them you have to create it. If there is a rule and you are using a version to edit your UN, make sure your version has the rule by reconciling with the DEFAULT.