Hi Pubudy,
This means you have not maintained the order type in any of the level.
Order type selection you can maintain in material master/MRP group/Plant parameter level. And the priority is to MM then MRP group then Plant parameter.
Material master maintain:
In material master you have to assign the production scheduling profile where the order type is maintained. Before that you have to maintain that profile in OPKP.
If you don't want to maintain the profile in material master, then you can maintain production supervisor. Because prod scheduling profile can me maintained in production scheduler. If scheduler is assigned in material master then it can fetch prod scheduling profile and intern order type.
MRP group maintain:
If it is not maintained in material master system will check in MRP group and same has to be assigned in material master.
By OPPR you can maintain it.
Material master view
Plant parameter maintain:
If system does not get it system will check in plant parameter.
in OPPQ it can be set.
You can set MRP group and plant parameter from OPPE also.
If there also it is not maintained system is helpless 
. Then you have to put manually.
Refer below link also.
Production Order Type Selection - ERP Operations - SCN Wiki
Regards,
Krishnendu.