I think the error might be misleading in this case. You don't include AS
when specifying table aliases, i.e.:
SELECT E.EQUIPMENTID, E.EQUIPMENTDESCRIPTION
From EQUIPMENT E
left outer join EQUIPMENT_CHECKOUT EC
on E.EQUIPMENTID = EC.EQUIPMENTID
WHERE EC.EQUIPMENTID is null
BTW: in Apex, you can try SQL statements in the SQL Commands window (in SQL Workshop) which usually gives better syntax error info.