I am not sure what you are trying to do with your query, but the syntax you are using looks like a linked extension field. E.g you are using a field from the Request on a task screen. If you wanted to grab this value I would use a subselect. Something like
:lHISTORY_DESC + COALESCE((SELECT VALUE_TEXT FROM SU_EXTENSION_DATA WHERE EXTENSION_FIELD_REF = 500221 AND CORE_ENTITY = 2 AND REF_NO = CR_TASK_HISTORY.REQUEST_NO), '')
:lHISTORY_DESC_HTML + COALESCE((SELECT VALUE_HTML FROM SU_EXTENSION_DATA WHERE EXTENSION_FIELD_REF = 500221 AND CORE_ENTITY = 2 AND REF_NO = CR_TASK_HISTORY.REQUEST_NO), '')