HI wang
Please could you check the below things and get back .
1. It could be that the PO for whom you are trying to GR is fully GR'd so there is no more GR possible .
2. It could be the PO which you are trying to GR is 'Locked' and hence not possible to GR
There is also a way you could enhance this message let me know if this seems to be the issue so that we can check further