Do you have OpenMRS custom validation schema enabled for your source?
Currently, setting this after a source has concepts in it can takes a while (for a small dictionary with a few hundred concepts, applying this schema should only take a few seconds; for a large dictionary like CIEL, it can take 20-30 minutes). This is because the concept validation rules are checked for all concepts before the custom schema is actually changed. The UI is currently designed as if it’s an instant change (change the dropdown for schema and click the save button), but the operation takes a while. So, you have to edit your source, select the OpenMRS custom validation schema, save the change and then patiently wait (I’d suggest 15-30 minutes) before checking to see if the change to the new schema was successful. If the OpenMRS custom validation schema setting does not persist (reverts to None), it means there were validation errors and you’ll need to reach out to the OCL team via Slack to get the report (a JSON output listing specific validation errors encountered), address those issues, and try again.
If there are validation errors in the source, the OCL module in its current state will probably not provide very gracious/informative hints into the errors. If you have the OpenMRS custom validation schema set on your source, it will ensure none of the errors are validation issues and prevent you from introducing new ones in OCL.