Hi Sergio,
Item master data form => inventory data tab, do you set G/L account by Item group or Warehouse?
if you set by Item group: Administration -> set up -> inventory -> item group -> accounting tab. Please set G/L account for item group of item
if you set by warehouse: Administration -> set up -> inventory -> warehouse -> accounting tab. Please set G/L account for warehouse of item.
I hope this can help you,
Hang
