It's possible that whilst your products don't have variants attached, they do incorrectly have variant options and attributes saved in the background.
For example your product may have had 'size' and 'length' as a hidden attribute.
- Vend for Mac or PC
- Vend for iPad
This issue can be caused by incorrectly importing a CSV or by a third party integration managing your products.
1. Click Products
2. Click Export
3. Open the CSV file in Open Office
4. Look for any products that contain a variant_option_X_name without a variant_option_X_value or vice versa.
5. Replace the incorrect field with the word 'null'
6. Save your CSV file
7. Click Import
8. Attach your CSV file