ETL+ Extract SQL Statement: CONVERT()
CONVERT() is specific to T-SQL. CAST() is ANSI standard.
The CONVERT function is specific to T-SQL and is not part of the ANSI standard.   If you require a function that is platform agnostic then you would be better off using the CAST function as it is compatible with other database platforms.    SQL Convert Function Examples and Alternatives [mssqltips.com].
Current DateTime in YYYY-MM-DD HH:MM:SS Format
SELECT
    [column1],
    1  AS  [column2],
    CONVERT(varchar(30), GETDATE() ) AS [column3]
FROM
     [sourceTableName]
SELECT
    CONVERT(varchar(30), GETDATE() ) AS [column3]
FROM
     [sourceTableName]
The SQL statements and syntax used in the Extract SQL Statement panel must work with the SQL syntax supported by the ETL+ Source Driver used to connect to the associated Source Object.