Get rid of the underscores.
select @beginDate =
CAST(CAST(((((MONTH(GETDATE()) - 1) / 6) * 6) + 1)
AS VARCHAR) + '-1-' + CAST(YEAR(GETDATE()) AS VARCHAR) AS DATETIME);
select @endDate =
CAST(CAST(((((MONTH(GETDATE()) - 1) / 6) * 6) + 6) AS VARCHAR) +
'-1-' + CAST(YEAR(GETDATE()) AS VARCHAR) AS DATETIME);