Hi Hasintha,
As already explained by Experts "R.Brahmankar & Mariano", MRP and availability checks are not related.
MRP is used for planning of finished materials as well as lower level components. Based on various parameters maintained in customizing / master data, system decides the shortage quantity and creates procurement proposal.
Availability check is generally carried out later, to check if sufficient stock of components is available for consumption / usage.
You can carry put ATP check in planned orders change mode or using transaction MDVP to get missing components.
Regards,
Rohit