Issue with Account Assignment PO for Account Assignment Category M

Hi Xperts

We have a scenario where we create an Export Sales Order from which we are trying to create an STO with Account Assignment Category M and Item Category U for Material.

We are using Inventory GL to have accounting impact on direct consumption where Sale Order is also my Accounting Assignment Object.This G/L is not allowed for manual posting.

Surprisingly , out of 10 processes , in one process system is throwing an error that "Direct Posting Can't be made to the GL : XXXX under Company code : XXXX"

What would be the reason for this.