"Serialization not possible for material XXX" when trying to PGR for an IBD

Hello,

We are getting below message while doing PGR for an IBD:

'Serialization not possible for material 56P4102'

IBD# 180434423 is directly created from an sub contracting PO# 4500195988

There are also 12 to 13 items along with this material and IBD is still open.

If you need more details please let me know

Can you please let us know what is the root cause and how it can be overcome?