Hi,
If you want the user to input the delivery date, then you may check the following option.
1. Go to ME51N and go to personal settings and go to default values tab.
2. Maintain the delivery date as any past date as shown below and save.
3. Now, go to the path: OLME - Environment Data - Define Attributes of System Messages - System messages. Here, maintain the message ME 039 as error message E as shown below:
4. Create a new PR in ME51N and check the system behavior.
System will always propose the delivery date mentioned in the personal settings (even though you maintain different PDT or purchase processing time). Since we maintained a past date as the default date, system will propose the past date and immediately the error message ME039 will be triggered. So the user will be alerted and he has to change the delivery date.
Please note that the personal settings is at user level and you have to maintain the same settings for all the relevant users.
If the above option is not suitable, you may go for development. You may check the BAdI: ME_PROCESS_REQ_CUST
Regards,
AKPT