Cada tabla derivada debe tener su propio error de alias

Necesita un alias para la subconsulta y debe aplicar las condiciones a las dos consultas que une:

SELECT MAX(DateTime)
FROM (

  SELECT DateTime
  FROM Class_Searches
  WHERE User_Email = '[email protected]'
  AND DateTime > NOW( ) - INTERVAL 30 DAY

  UNION ALL

  SELECT DateTime
  FROM Book_Searches
  WHERE User_Email = '[email protected]'
  AND DateTime > NOW( ) - INTERVAL 30 DAY

) AS x

o devolver datos para que pueda aplicar la condición en la consulta externa:

SELECT MAX(DateTime)
FROM (

  SELECT DateTime, User_Email
  FROM Class_Searches

  UNION ALL

  SELECT DateTime, User_Email
  FROM Book_Searches

) AS x
WHERE User_Email = '[email protected]'
AND DateTime > NOW( ) - INTERVAL 30 DAY