After updated Version of ERp we are unable to edit in item codes. after making variants nothing has to be edited or changed in code screen . Please check the issue attached picture is there for your reference
For the variant item, Item Code will be generated based on Template Item’s Code + Abbreviation of attributes.
In the variant item, expect Item Code, now you can edit other values like Item Name, Description etc. We have recently pushed a fix for the same. Please check and confirm.