This step is optional. You will only need to set up if you adding Manual Items when creating Credit Note
Click System Configuration
Click on the small arrow beside General Settings to expand the dropdown
Click Manage UOM
Click Create New
UOM Code: example ITEM, PCS, KG, SUM
UOM Name: example ITEM, PIECES, KILOGRAM, SUM
Description (optional)
Once done, click Create
The UOM created will appear here