The External Reference field is not editable in Over-the-Counter sale transaction.
Reproducing the Issue
1. Go to Over-the-Counter sales work center.
2. Create a new Over-the-Counter sales transaction from Common Tasks.
3. The external reference field is grayed out.
This is the designed behavior of SAP Business ByDesign system.
The field External Reference is not supported in the Over-the-Counter Sale transaction.
As a workaround you can create an extension field in the Over-the-Counter Sale UI to add the reference purchase order ID.
Additionally you can raise a new functionality request or can contact SAP Service Center for customization.
KBA , SRD-CC , Cross Components , How To