Dear,
In MRP 1 view of your material master you can define the rounding value as 24 so your all PR will generate in multiple of 24 only. Also define the alternative unit of measure as 1 Carton = 24 PC and in Purchasing view of your material master use purchase unit of measure as Carton so system will generate the PR in Carton. Please try the same and come back.
Regards,
R.Brahmankar