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.
(You can auto filter the columns to check for any text in each column without having to search manually)
5. Replace the text in the field with the word 'null'
(if there is no text in the field you can simply move on to step 6)
6. Save your CSV file
7. Click Import
8. Attach your CSV file