Hello Amit,
Is same order type and plant combination giving different settlements type of different materials under same plant?
Please cross check OKZ3 setting for order type and plant combination for Default rule.
also check "Cost collector" indicator is set?
Also check OPJH for settlement profile for order type.
Howeverr, the settlement type PRE is permitted when changing an existing orders.
Best Regards,
R.Brahmankar