Below are step to setting running number in Sales Invoice :

Scenario 1: If Your Sales Invoice Running Number Already Exist

Step 1: go to System > Administrator Menu > Running Number Setting

Step 2: Choose Sales Invoice >> Refresh >> Click Edit Item >> Change Date To >> Click Add Item

Scenario 2: If Your Want Add New Running Number Sales Invoice

Step 1: go to System >> Administrator Menu >> Running Number Setting

Step 2: Choose Sales Invoice >> Refresh >> Change Date From & Date To >>  Type your Prefix and New No >> Click Add Item