Calculate time difference in minutes
Checking the docs for Amazon Redshift shows this:
DATEDIFF ( datepart, {date|time|timetz|timestamp}, {date|time|time|timestamp} )


datepart: The specific part of the date or time value (year, month, or day, hour, minute, second, millisecond, or microsecond) that the function operates on. For more information, see Date parts for date or timestamp functions.



Specifically, DATEDIFF determines the number of date part boundaries that are crossed between two expressions. For example, suppose that you're calculating the difference in years between two dates, 12-31-2008 and 01-01-2009. In this case, the function returns 1 year despite the fact that these dates are only one day apart. If you are finding the difference in hours between two timestamps, 01-01-2009 8:30:00 and 01-01-2009 10:00:00, the result is 2 hours. If you are finding the difference in hours between two timestamps, 8:30:00 and 10:00:00, the result is 2 hours.



date|time|timetz|timestamp:  A DATE, TIME, TIMETZ, or TIMESTAMP column or expressions that implicitly convert to a DATE, TIME, TIMETZ, or TIMESTAMP. The expressions must both contain the specified date or time part. If the second date or time is later than the first date or time, the result is positive. If the second date or time is earlier than the first date or time, the result is negative.

So, your query should be:
select 
    email,
    createddate,
    lastloggedin,
    datediff(minute, createddate, lastloggedin)
from udb.user
where createddate &gt;= '2019-09-01' and createddate &lt;= '2019-09-30'