Трябва да опитате с малки букви за идентификаторите на таблица/колона (като в from /inner join клаузи):
SELECT entertainer_id,
entertainer_groupname
FROM casestudy_entertainer
INNER JOIN casestudy_availability ON casestudy_entertainer.entertainer_id = casestudy_availability.availability_entertainerid
INNER JOIN casestudy_calendardates ON casestudy_availability.availibility_calendardateid = casestudy_calendardates.calendar_id
WHERE entertainer_type = '&Entertainer_TYPE'
AND casestudy_calendardates.calendar_date = '&Event_date'
