My process for successfully utilizing the Error Inspector for validation rules is as follows:
1. Create Validation Rule (automagically creates the 4 validation features GDB_ValidationPointErrors, GDB_ValidationLineErrors, GDB_ValidationPolygonErrors, & GDB_ValidationObjectErrors in the egdb)
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2. With a map with the data with validation rules applied, add the 4 validation features to the map to publish
3. In the Share As Web Layer Configuration tab check 'Validation', Publish
4. Bring in service in from Portal
5. Right click on a feature with validation rules and the Add Error Layers is available
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