When you try to post a GR against any PO it says that the PO does not contain any items. 
But when you display the PO it shows the items in it. 

There have been several reasons which can be attributable to this problem - 

1) This PO is subject to release strategy and since the final release was not made, this PO is considered as 'blocked'. 

2) You have been using the confirmation control category where in respect of the last confirmation category key, the GR assignment checkbox is flagged. In this particular case, GR is only be posted if the confirmation is not maintained. 

3) Early delivery date ie. the GR date is coming earlier than the PO delivery date. 

One other nasty thing might have happened (mostly in MIGO): 

You have accidently changed your default movement type from 101 to 102 during your last movement. 
You will experienced this in some cases before installing hotpackage which shows the movement type on the MIGO screen. 

Do you have a SAP MM Question?

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.sap-img.com