If your base was copied from the Merch Wizard starter template prior to September 2019 you'll need to add 3 fields to your Designs table so that Merch Wizard can populate them with your cloud storage links.
There are only 3 new fields required and all of them are of type URL, so please open your Designs table and add these fields by right clicking the MerchPopsocket field and clicking "Insert Right" :
In the end it should look like the below screenshot. If they're in a different order it doesn't matter as long as all 3 fields have been added you're all set ?
You can see a video of these steps below: