I can successfully perform the query

SELECT * FROM `mos_availability` a WHERE NOT EXISTS (

SELECT *
FROM mos_users b
WHERE a.user = b.id)
But

DELETE FROM `mos_availability` a WHERE NOT EXISTS (

SELECT *
FROM mos_users b
WHERE a.user = b.id)

Says that I have an SQL syntax error.

Can anyone spot what it is for me?

TIA
Regards
Paul