How to disable the releasing button for the 2nd level of Purchase order

Rakesh,

I believe workflow is involved in your case. As said earlier, better to bypass ME29N irrespective of ECC version.

To achieve this you can use a user decision dialog step for approval and in case approver approves, a background step for release with particular release code. For this background method implementation, you can refer the method SINGLERELEASE of BUS2012.