Getting error- is not an internal table “OCCURS n” specification is missing in SAP method

You need to define the et_flights parameter as of type SFLIGHT. As per method defined, you have this type as structure type and also declare transparent table SFLIGHT at the same time.

You should use an already available dictionary table type with row structure of SFLIGHT for et_flight.

You should declare et_flights inside the method in class definition as a private member or as a return value of the method.

class myclass definition.
   public section.
   types ty_mytable type standard table of sflight.
   methods mymethod exporting mydata type ty_mytable.

class myclass implementation.
    method mymethod.
       select * from sflight into table mydata.