There could be a situation in AX2012 when you have created Purchase order and saved it (Project ID is unknown in this stage). After that if you want to enter Project ID to a header level or line level- it is not possible, Project ID field is greyed out.

The conclusion of this behavior is that this is how project Purchase order is currently designed:

- The Project ID on the PO header shouldn't be editable when a PO header is created without a project.

- The Project ID on the PO header will only be editable when a PO header is created with a project.  The project ID on the PO header also cannot be cleared when a PO header is created with a project.

This design is done due to some consequences changing project ID in Purchase order.