I use Attribute Assistant in ArcMap to concatenate 2 fields into 1 field. The data is on an Enterprise server with the feature class coming from SQL server. I'm currently trying to duplicate this in ArcGIS Pro using Attribute Rule. I entered the Arcade statement to concatenate that works. I get an error message when I go to save the rule ERROR 000464: Cannot get exclusive schema lock. Either being edited or in use by another application or service. This layer is versioned and published to ArcGIS Online. It would not be practical that every time one of my users goes to edit this layer I have to remove all locks. Is there any work around for this? Attribute Assistant in ArcMap works perfectly. Thanks for the help.
If I understand your question, I ran into something similar, if not identical, when I first started working with attribute rules. From your screenshot, it appears the error is happening when you go to save the attribute rule itself (not when someone does something that triggers the rule). There can be no connections to the feature class for which you are creating the rule - even having the feature class added to a map in the same aprx as you are creating the rule will give you the schema lock error.
From your screenshot, it appears that you do not have the rule disabled - that would be the first thing I would check. I also see it is only triggered upon an insert - perhaps that is where the problem is. Are you creating a brand new feature via an insert, or are you changing attributes on a feature via of an update? Beyond that, I got nothin'.
When you say "Published to ArcGIS Online". Are you creating a hosted feature service layer? ArcGIS Online does not support Attribute Rules.
Once a layer is published to AGOL, there is no connection back to the original layers. It is a copy of the data with the supported properties.