SKU use only Classification in product master .Without classification not use SKU. and SKU add in new year and new company.

Step 1) Go to Environment Setting > Product .
Step 2) Then select YES in the SKU No Required.
Step 3) Then Make a new Product and select the Classification after that show option Required SKU No and select that.
Step 4) After that go to Master > Product > SKU Master
Step 5) And select the combination which you want to use in SKU and then make SKU.
Step 6) Press Save (F2).