If an order type is not displayed in the drop-down field Order type in the Placement records then this order type has not been added to the Placement's mediakind in the General Settings. 

Depending on what kind the mediakind (media type the Placement is (e.g. TV or online), the order type needs to be added to the according mediakind record.

The mediakind record can be found in the General Settings.

Most used mediakinds are linear or non-linear as media type TV for example is linear. In the following screenshot the linear record has been accessed and displays the vector field where the missing order type must be added.

As soon as the missing order type has been added and saved, it will be displayed in the Placement record's Order type field inside the drop-down to chose from.

Note: This description depends on the condition that the order type already exists. If it does not exist at all in Mydas yet, it needs to be created first...here: