Here is a series of steps that you can do to upload your predefined SKU:
Click on the Products tab.
On the top right hand corner, you will see a drop-down arrow, please click on that.
A dropdown button should appear.
Select the option: Import Products.
Once uploaded, you should be able to see all your preloaded SKUs.
Drag and drop all the fields that are related and relevant to your column types and click import.
This will allow you to be able to upload your preloaded SKUs.