you can use transaction ME28, this can give information of pending purchase order for release on release level.
but this transaction is used for releasing the collective release for purchase order. this is not an report.
if you want transaction for reporting purpose only, you need to create a report or query on EKKO table. based on release indicator.
T_Code ME80FN, Select Document Category as "F" , click on Dynamic Selections ( Shift+F4), select Purchasing Document Header, Double Click on Release indicator , select Blocked and Execute.
You will get list of Unrelease POs, save varient for further transaction