At the left side in Menu Bar, there is option of button "Setting". You can find setting option in "My account" as well. We have to click on it.

After clicking on Setting, we have to go in the "General" option.

Here we can do enable or disable the "Unit" function.
If we will do it enable then Unit option will show while creating invoice and if it is disable then it will not reflect.
By selecting this option, if we are doing sale of goods in KG, LTR etc then we can select this option.