Hello Kiran,
I think you have turned off the flags in transaction OVZ8, but if you do F1 (help), you will see that is for 'Sales' part.
May be you could the settings in below mentioned customizing path.
SPRO >SAP Reference IMG >Materials Management ->Purchasing >Purchase Order > Set up Stock Transport Order >Assign Delivery Type and Checking Rule, and maintain the new "Availability Checking Rule column".
Let me know in case of any queries.
Regards,
Bhavin