There are a few reasons for getting this error.
Most commonly, it's because the Table Names, Field Names or Field Types has been changed from the original Merch Wizard Starter Template base.
If you have changed any of these, please change them back to the original default settings. You can compare against the Starter Template or copy a new version of the base and start afresh.
Here's a short video explaining how to identify which field is giving the error
If you are still having problems, then create a support ticket and we can look into it further for you.