I'm trying to model the electrical model for our own location.
The model has only 1 Tier. I am running the Subnetwork Update tool and it updates without any problems.
Then I add Tier 2, after this tier is added, the Subnetwork Update tool starts to give an error.
When I run the Subnetwork Update (Tier1) tool it gives the error "No valid subnet controller found". All I did was add Tier2.
I couldn't find what was causing this error although no action was taken outside of this workflow.
I am using ArcGIS Pro version 2.9.1 and working on File Geodatabase
I guess, your scenario is like this;
You have got a subnetwork&line and you are removing its controller. So after once you want to update the existing subnetwork&line, it says that no valid controller found.
Of course this can be one possibility : )