Problem statement: 

Transaction code is coming empty at POS for the Combined transaction (Sale and Refund in one transaction) refer below image:

Cause: 

Default Document number series is Not created, Inactive or Expired.

How it works: 

When a SALE and REFUND are done in a single transaction then POS picks the “Default” document number series and if default document number series is not created, Inactive or expired then “Transaction Code” will remain empty.

Resolution:  

To resolve the concern need to Create “Default” Document Number series, if it is already created then Make sure it’s not “Inactive” or “Expired”.