Which Pro version?
Does the class have batch calculation or validation rules (check the datasource to see if GDB_Validationxxxx classes exist)
Which data source is this ? (filegdb/mobilegdb/enterprise feature service?)
Note that the add error layers only work through mobilegdb/filegdb and in feature services after consuming the feature service which has a validation capability enabled.
this video might help https://www.youtube.com/watch?v=HFHJFCn2E1M
I am currently working in ArcGIS Pro version 2.9 and am experiencing the same issue. I watched the attached video in hopes of being able to correct the issue of not having the option to click the 'add errors layer' but the steps that were shown did not assist me. Is there another process of enabling this option?